I could see them charging you to fix the SR9 if there was a problem and it was Ghost's part's fault (they'll probably put a stock part back in and not return the Ghost part). However, if you do have it installed and there is a problem that might be related to the Ghost Trigger, especially if it develops immediately after it's install, call Ghost! From what I've read and experienced first hand (just in ordering), they seem like they're a small business, but with Customer Service to match or better what Ruger can do. Don't send them your pistol! Just explain and you'll probably get a new part no questions asked.
Any part that modifies the workings of anything should and most definitely will void any warranty. Ruger doesn't really come forth and say that there is a warranty. They do offer good and reasonable service no matter what we seem to do to these things, our fault or not. If it's their gun and their parts that aren't working, they'll fix it....or try to do so.
If you want the Ghost Connector, get it. If you start having problems down the road, try swapping back to the Stock Connector. If the problem persists, call Ruger. If it goes away, go back to the Ghost and if it comes back, call Ghost about it. Just my .02
Josh