You realize that the gargants being slow has nothing to to with the format of the list. It's the stats they've always had.

The stompas, lootas, killa kans, supa-stompas, gargants, great gargants and mega gargants all have a move of 15cm.
The only units that has more movement is the battle fortress (which in their current form can't be used by the Lootas since the formation is too big to fit in a fortress) and tve flak wagons you can upgrade the formations with.

So the list is slow. Veeeery slow. It's one of the drawbacks of the list. And it's very crippling to the goal to make it competetive since the game mechanics makes it kinda hard to be that without the possibility of moving around the table to grab objectives.

Theres a couple of ways to mitigate this drawback:

1. Put in more fast units into the list. This will make it more like the Ghazkuul list which isn't a great thing.
2. Make it possible for the gargants to move a little faster (a 20cm move isn't that fast by the way, do you guys think of a Gorgon as a fast unit too??) by introducing an upgrade like the reinforced boilers.
3. Make the Battle fortresses more usefull by making sure the Lootas can actually fit into them.
4. Put back Landas or even Drop Rocks into the list and make air assaults possible again. This option make the list waaaay to OP IMO since gargants and air assaults deep into enemy territory is a crazy powerfull list synergy.
5. Other options i haven't thought of...

As I said earlier, i agree that some kind of risk could be cool to include into the boilers upgrade, it feels very Orky. But to make it so bad that it stops a 6-800 formation from moving or blowing it up is just too much.

Hi, I'm planning a gargant army of my own and had some thoughts.

1. Why not a kill kan/dreadnought unit? If the idea is to be a walker based army?
Normal mob - 5 kans (25 each) or dreads (35 each) and so on. Would be an alternative to fighta spam.

2. Why not allow stompas to have 0-1 oddboys in the mob? Would perhaps make them more enticing.

3. Kustom weapon "Zupa Shook Gun". 45cm MW4+ TK(D3) hits bypass any power or void shields.
Read the fluff on shook attack gun to see the reasoning behind this.

4. The rules already allow for customization on the supa stompa (ccw or more dakka) why not go all the way, remove 2 weapons and allow 2 Kustoms? With hje appropriate points reduction of course.

5. I liked the idea of a supa stompa mob. Normal mob 1 supa stompa 325 points (allows 2 support options). With greater size allowing more support.

Just food for thought.