On a RS Type 56 with good compression a Modify S110 will put you right on 400fps. Maybe 5fps over, maybe 5fps under. If you want to err on the side of caution a Modify S100 will put you right in the 380's.
Some games are very strict on the max 400fps limit, others aren't so much.
If someone's going to charge you $15 to swap out your spring...go for it.