The warranty issue goes away if you send it to Arnel. I can tell you he does top notch work. You will have to send the gun off to someone to have it refinished as Rock Island does not have the facilities to it.
As far as parts any of the majors out there make parts for the hi cap. The only exception is the trigger, you will have to find a a para or a springfiled. Do not use a fusion as they sell triggers that only fit the STI 2011 and they will not fit the Rock, don't ask how I know.
I also use S&A meg wells on mine as I love the them. I like the way the even out the bottom of the gun/mag.
Grips are also problematic as RIA offset's the grip screw bushings just a fraction so some grips fit and others don't.
Good luck on your build.
Mike