Battlesuits cost:
I think 500 for the Boardsides is to high
you changed from 4.1 to 5.0:
TH +3 >>>TH +4
Points 350>>>500
thats to much I think 400 will do it
I loose every battle, I fought with the Tau, half of my boardsides in Turn 1

Tau Command units
For every Co. which is lead by a battlesuit command unit ,
I raised the cost up to 50 points
thats still cheap

Tigershark:
The railgun variant is to expensive and has a low AS
Please think about
this is the Tank Hunter Variant (like IL-2) why should it has a lower AS
should be AS +3

400 points/model is silly
compared with the drone carrier
the tank hunter variant is only slightly better armed
should be 150/model or 200/model

And 1 questions:

Am I right?
If the Manta/Moray failes its fixed shield save,
the shot will hit the hull
even with more activated shields?

That is worse than the eldar holofields
this means you can shoot down a manta with the first shot?

Should be:
if fails a fixed save the Manta/Moray will loose one shield
if it failes 2 it will loose 2

i.g. The manta is hit by 7 shots and failes 2,
it will only loose 2 shields

Juffo-Wup

You have to consider that the penetrating ability is very powerful and should cost accordingly.

Crissis suits low as a signature Tau unit that was their "really good value for the points" unit

The one thing that can be abusive with the Tau is their flyers.  I'm particularly cautious about flyers with long range shooting ability, as coupled with high altitude they can be nearly invulnerable.  The AX-1-0 has massive firepower with long range so will be very expensive.  It is probably most comparable with a Phoenix Bomber that has less 75cm shots and still costs 300 points.  Playtesting also showed it very often earned it's points back (it shredded many SHTs, Praetorians and the odd Warlord titan in its day).  I think if there is any argument to be made then perhaps the Drone Carrier should increase a little.

it shredded many SHTs, Praetorians and the odd Warlord titan in its day

We tried to offset the risk of abusive flyer armies with the Tau to some degree by limiting the effectiveness of the AA (a little) and giving their flyers low CAF.  Despite this, it is still easy to make a Tau army list that is extremely difficult to fight against if you don't know what is coming.  The smart Tau player can also surprise you sometimes by taking no flyers too.  

Deflector shields is the Tau thing so I'm fairly loath to change it.  I acknowledge it's not the best defense, but this was what was developed by the original writers.  I find Tau landing craft are more vulnerable to CC anyway.

Tau.
Hammerhead with twin linked plasma cannon
20cm/As+3/CAF+2
TL PlasmaC. AD.,4x TH.+4 TS:-1 Range 50cm

Hammerheadwith twin linked Burst Cannons
20cm/As+3/CAF+2
Tl BurstC. AD.6x TH.+5 TS.-1 Range 50cm or AD.6x TH. +4 TS.+0 Range 50cm

both variants with missile pods

Before the Tau codex  shows up,
I have some suggestions about Tau stats and point costs

Here is my proposal about Tau rail guns:

Light Rail gun: Range 75cm TH +3 TS -1 Pen +1..............Units: Bettlesuits
Medium Rail gun: Range 75cm TH+3 TS-2 Pen +2...........Units: Hammerheads,Tigersharks,Vektor Knights
Heavy Rail gun: Range 100cm TH +3 TS -3 Pen +3...........Units: Manta/Moray, Swordfish Super Heavy Tanks

Battlesuits cost:
I think 500 for the Boardsides is to high
you changed from 4.1 to 5.0:
TH +3 >>>TH +4
Points 350>>>500
thats to much I think 400 will do it
I loose every battle, I fought with the Tau, half of my boardsides in Turn 1

Crisissuites:
250 points is to low
I think 300/units is better

Tau Command units
For every Co. which is lead by a battlesuit command unit ,
I raised the cost up to 50 points
thats still cheap

Tigershark:
The railgun variant is to expensive and has a low AS
Please think about
this is the Tank Hunter Variant (like IL-2) why should it has a lower AS
should be AS +3

400 points/model is silly
compared with the drone carrier
the tank hunter variant is only slightly better armed
should be 150/model or 200/model

Please reactivate the Tau vektor knights!
Just replace the "flyer kit"
with a jump pack
similar to the eldar reverants one

And 1 questions:

Am I right?
If the Manta/Moray failes its fixed shield save,
the shot will hit the hull
even with more activated shields?

That is worse than the eldar holofields
this means you can shoot down a manta with the first shot?

Should be:
if fails a fixed save the Manta/Moray will loose one shield
if it failes 2 it will loose 2

i.g. The manta is hit by 7 shots and failes 2,
it will only loose 2 shields

If you are thinking about a Railrifle for the Crisis suits

Fair point but in the battles I have played with them, 500pts seemed about right.

If you are playing a pure tau force without Kroot, you're gonna have a very, very hard time capturing objectives and the Crisis suits with their hit-and-run ability allow you to do just that.

Every army needs a great-value unit and the Crisis suits are that unit for the Tau

I think they need to stay the same for the same reasons as above.

As zap sait, flyers with powerful, long-range guns are very, very powerful on the table.  They can be virtually impossible to kill at times.  I have found that the Railcannon Tigershark is well worth its points cost.  Hell, I take two.

The Vector Suits are still in the options book if you like using them.  And I believe I did remove the flight pack and replaced it with a jump one.

Yes.  The Deflector Shields.  I've never really been happy with them.  I am very tempted to scrap the whole idea of them and just give any Tau unit that has Deflector currently a fixed save.  In the fluff, Tau shields DO NOT opperate like Ork r Imperial shields.  They don't absorb incoming fire.  They deflect it away from the unit.  As a such I am tempted to simply give Deflector Shielded units a Fixed 2+ or 3+ save to represent the shield and if the shield generators are damaged, they lose the shield save.  No keeping track of how many shields you have.

I am also not happy with how the Markerlights work.  I am looking at how to re-visit them and make them more useful.

The Broadsides are fantastic, but 125 points a stand is a lot.

but if as JW says if it is not a reflection of the (40K) facts then I'm happy to leave it.

Flyers - I'm pretty comfortable that these are balanced and correctly costed already.

 Vektors - yeah, right

Couple of new Hammerhead variants to reflect available FW models - these look ok to me