STI Six-speed DCCD 2WD Conversion Issue

[Scargo]

Would a 2008 GR 6 speed work as well or is it a completely different setup?

It can work. The part FFR sells is for a GD (pre-2008) transmission. The DCCD (center diff) is different for the GR's. Note that several people have welded the parts together rather than bolting. You cannot use the FFR piece and bolt it to the later model design of diff. Diffs are not interchangeable.
I believe that this issue is discussed and described in this thread, if not another on this 818 forum. Try searching.
I have a GD six-speed with close-ratio gears, built-in oil pump and an OS Giken LSD setup specifically for the 818 and racing. If this is of interest to you or anyone else PM me. I am thinking I may go with a five-speed. I also have the FFR adapter for the six-speed. The tail end is not modified.