How much difference will non cat downpipes make over stock downpipes. Also new intercoolers are being installed as well. Also I to go with the new downpipes I have 2.5 inch test pipes and HKS hi power exhaust.

More noise and more pollution. Also perhaps slightly faster spool, PERHAPS.

Spool response increased quiet a bit even from JDM down pipes to 2.5" SS down pipes. IIRC shifted my graph to the right by 500-600 rpm. USDM precats(downpipes) are much more restrictive than the JDM 1/4 DP. I did not notice any exh noise changes. So you should notice a difference, especially lower end RPM.
When I went from 2.5" DP to 3"; Very little change in overall power but spool noise was noticeable.
Note: this was on OEM turbos so maybe the 3" dp was over kill for the smaller sized turbos

Awesome thanks for the comments guys cajt wait to get it all put together....if I can figure out a way to post some pics and vids i will