[Legs]

Although your spring certainly can affect your ROF, ROF is more affected by battery size and rating.  A "standard" battery will be bogged down a bit, creating a slower ROF, if you upgrade to a stronger spring...but if you upgrade your battery power your ROF will jump considerably.  Generally speaking, you "upgrade" both.  A stiffer spring requires a good battery to utilize it.
As for your FPS with an M100, there are a lot of guys here with greater knowledge than me in that category.  An M120 tends to grant users an FPS rating in the mid-to-upper 300's on average, I think...so an M100 should be in the lower-to-mid 300's.  Paco?

"You are, what you do, when it counts."
-The Masao

"Get on that fifty!  Someone get on that fifty!"

[Paco]

An M100 is theoretically 100 meters per second.  An M120 is 120 meters per second and so on.  ROF for an M100 wouldn't be affected too much, even if you use an 8.4v battery...  It wouldn't drop enough for you to really notice it.  Your range depends on the gun you install it in, barrel length and any other upgrades performed on it.

[dvlax]

if i were to install an m100 spring, what would the fps range be? and how much would it affect the rof?