I have a 2008 GXP and it idles at 600 rpm. I also drive a 2008 Suburban with a 5.3 and it idles at 600 rpm. it is very typical for V8 vehicles. I would check for an exhaust leak since you just changed your downpipe, you may not have one, but with it is best to eliminate the simple stuff first. the hard shifting could easily be a tuning problem, but there might also be a transmission issue exacerbated by the tune. might be best just to put it back to stock.