I don't think Imuran will cause any problems with your IBS. Imuran is really great at putting people like myself with UC and others who have Chrohn's disease into remission.
I think you'll see a postive change with it. I know that it has a very long list of scary side effects and that did make me hesitant to take it. That being said my doctor explained it to me that those do not usually happen for people taking low doses (like people with CD and UC) it's mainly for people using such high doses to prevent organ transplant rejection. Well that reassured me a bit and I've been taking it since Dec. 2004 and have been in remission pretty much since starting it. I haven't had any adverse GI effects as a result of taking it.
Maybe talk to your doctor about it's success rate with treating RA.