http://www.airsoftparts.ca/news/modifyspringtest.pdf
bit of a hit and miss but if you follow modify's upgrades, you
could get similar results.
I'm putting an s100 in my MP5 but it will be strictly used for outdoors. I have put in an s90 in my TM AK that is bone stock with a 229mm TB but will have to chrony it to see what it actually shoots.