Hey man, I'm planning the same thing, I figured that if I was getting 320fps with an m90 in an MP5k, then I should be able to get a similar fps with an AR pistol. I'm doing this for the exact same reason too, simplifying my gear to use the same magazines.
Check out my other thread, I'm looking to have an AR pistol holster made.
http://airsoftcanada.com/showthread.php?t=60190