With the stock KWA (guessing M4)...I would guess (everything through the internet is a guess
) at putting in a Modify S90+ spring. Although the "strict" conversion of 90m/s only works out to 297fps...with the setup of that mechbox/AEG you'll probably hit closer to 340.
I've seen plenty of mid-upgraded (i.e. good piston heads, air nozzles, tightbore and bearing spring guides) that have shot 360-370fps w/ 0.20g bbs with a M100 spring.
If you want to be sure of being under the MAX limit (and it is a limit no an objective) of 350fps...err on the low side. For most indoor engagement ranges even 330 is plenty.
But buy a Modify S100+ as well...you never know for sure until you chrony it!
Tys