does the downpipe make a power difference. im not going to go with a cat back exhaust either just a axle back

 

Yes the downpipe will make a difference.
First it will allow the turbo to spool better due to less back presure after the turbo. Even with a DP and the stock exhuast there is a difference. Plus you get the added bonus of hearing the turbo spool (louder) with a DP.

Please use the search feature for more information then you could possibly want on all the downpipes out there.