Most places will not allow 400 for indoor. In reality, 350 is more than enough for the usual indoor paintball places. You won't notice much difference in your ability to hit something with 400 instead of 350 in such short ranges.
Heck, I played indoor games with a TM MP7 AEG that shoots 250fps and I find it plenty. Its the hopup system and good, heavy BBs (0.28g or higher) that matter more than just high FPS
H&K G36c (KWA) / Glock 18c (TM) / Kimber LAPD SWAT Custom (TM) / Mossberg M500 (ACM)