Here's the set up I plan on running a USDM type r header, hi-flo cat, type r exhaust on my gsr. Is the Usdm header a worhwhile upgrade over the USDM gsr header or am I better of with the JDM 4-1 does anybody have this set up currently

 

The USDM ITR manifold isn't going to be a worthwhile "upgrade" from your GS-R unit. You'll be much better off with the JDM 4-1.