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My understanding, and someone can correct me if I'm wrong, is the rough rule of thumb is a drop of ~10fps per 0.02g increase in BB weight. So a gun firing 400fps with .20g will fire roughly 375fps with .25g. Note that the effective kinetic energy doesn't decrease, in fact it will generally increase. So a gun firing too hot will still be firing too hot, no matter what weight of ammo you use. Limits are intended to be with .20g bbs even if no one actually uses that weight except for CQB and maybe not even then. Generally .25g or .28g are used for outdoor games.
Since you stated the mech box will fire 400-500fps that's too hot for practically anywhere. 400fps is the generally accepted maximum for outdoor games, with 350fps being the max for CQB. Note that I said maximum, not desired. My little tm mp5k-pdw is stock at 280fps and does decently against assault rifles firing 100fps more. Somewhere in the middle of 350-400fps (with .20g) is probably where you want to be aiming for unless you plan on using the weapon for CQB, then aim for the middle of 300-350fps
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