Alright I have done this EXACT thing. First off, the wrx midpipe is too short to bolt on and the flange doesn't fit on our cars. So what I did was cut the WRX midpipe just shy of the flange before the bend. Cut the stock RS midpipe just after the flange. Add about a 14 inch section of 2.25" pipe to the modified WRX pipe and then take the stock RS flange that was just cut off and weld them all together. In all very simple. Then the WRX axle back just bolts up to your new custom midpipe. It cost me $60 in all.
AS for performance, well it is a little louder but has a little whistle sound when you turn it off. If you can live with that it is all good. The car felt stronger after the exhaust was put on and it looks stock. I just went thru DEQ with it and they didn't even look at it
Score for mods