Well 400 was a random figure - in part to show just how expense they have to be before you would stop taking them. At what figure would they become unlikely for you? For me I would be unlikely to take them at 350 - though note I could still do so, its not completely out of the question. I would instead though probably rely on pred a's and hunters and terminators instead at that point.
300 would indeed make no difference, I would just be 50 points down elsewhere (and no 3 warhound armies anymore, but they in and of themselves aren't a problem).
I would think as a minimum they should get an instant kill critical. This doesn't affect me taking them, it just makes it feel a lot better when you roll as the opponent as currently its time to crack the warhound critical joke.
Then if you really wanted to change their attractiveness over other titans and other things in a list you would need to hit the points/stats (I don't know if you could do it with points, hopefully yes, but is their a point where they go from must have to don't touch? I would have no problem with trying things like init 2+ which would hit a key plus point with them, that of being extremely reliable).
As to the seeing something else dominating. Great. Surely its better to have something else dominate rather than these things which have had their time in the sun for I think too long? Even better if its an element from the appropriate race.
the fact you don't normally take them also means the debate isn't really aimed at you, rather its looking at list tourney balance and flavour - I for one am sick of seeing warhounds in top Imperial lists but its hard not to take them. Saying that Ii was all set to launch my Chimera horde into open war, not saying it would have done that well though and deep inside I know replacing the third chimera company with 2 warhounds makes sense, even when they are inferior to the marine ones

Note I think this is different to the compulsory thunderbolt squadron. Hard to say why but having planes overhead when you need them seems a lot more likely then always having warhounds around. Maybe its my other gaming prejudices that tell me getting an A-10 when you need it is a lot more likely than having an Abrams permanently attached, since it can just fly over and give support to a range of armies operating in an area.
That and the fact they seldom dominate, never 'win' games (aren't on the ground), can be dealt with easily and don't reward spending loads of points on them.
Edit - and what Gavin said above - I don't think the marine lists were designed to have a reliance on warhounds, their constant presence has developed over the years. It is a good point though - does a non warhound list fall too much in power? Can land speeders, predators and land raiders make up for it?