There are many differences. The GTi-R is very different from all SRs period. The head is unique to the GTi-R, it has solid lifters, quad throttle body, and has a compression ratio of 8.3:1 instead of the 8.5:1 that all other DETs have. So to make it short, nothing is interchangeable between the GTi-R SR and the RWD DETs other than very minor things. As a matter of fact, the RWD SRs don't share too much with their AWD/FWD brethren.
There are other kind of DETs too, other than just the GTi-R. I recommend that you use the search button and look up the archives.