There is a pretty massive tonnage increase between the 380/390XL and the 400XL. I'd have to shave heatsinks and ammo to fit it and it seems like a good balance right now, not to mention the speed increase from 390 to 400 is pretty negligable. As it is the Battlemaster rolls at near 80kph which is Chuck Yeager speed for something that massive.
I'm running 400XL in a Banshee 3M and Atlas Boar's Head but my other XL assaults are running XL350-390 ( Victor, Awesome, Battlemaster ), and 325XL for Highlanders.
I would estimate the times I die due to CT / XL kills to be one game in 10 vs regular CT kills. The thing is pretty survivable and tanky.
The Battlemaster 2C is even more tanky with structure buffs- that thing with 4 large pulse lasers is scary. Played some rounds last night and got 5 kills several times with it.