Yeah, an RIS with a vert grip is very different then the stock handguard.
400 is the absolute upper limit for most fields so as previously said, a limit not a goal. 350-380 is a good target to aim for but 300 will do fine for quite a while unless you're playing in areas with large flat open spaces (ie 100 ft or more), in which case the little extra range you get with the increased fps would come in handy. As to whether it would strip stock internals, that depends on the internals, it's different from gun to gun and manufacturer to manufacturer. Generally anything over 350ish and you should consider upgrading the internals more then just the spring. I'm not really the one to answer questions about that though, I don't know much about internal upgrades beyond being able to recognize a good parts list when I see one. My advice is get the gun first, play with it awhile and if you notice any shortcomings then you can start thinking about upgrades.