Ok well I gave my buddy a call and basically here's the breakdown.
Don't want to go smaller then 2535, anything smaller then the 2835 has to have some weldeing/custom work done. The "R' stands for response, and the "R" turbos have larger inlets.
Here are the max power estimates for the turbos. turbo/twin
2535 max power rating 340hp/680hp
2540 can't run on mkiv we need a T3 flange
2540R max power rating 370hp/740hp
2835 max power rating 400hp/800hp
2835R custom work needed not suggested
With looking at the 2835 numbers I make the estamte that the numbers are from well tuned and decently high modified cars. With that said I concur with HKS that the 2540R would make the ideal twin setup to run. Makes enough power with much less lag then the larger 2835 twins.
HKS only has a few turbo-less twin kits available in the U.S. so move quick. You can order a custom twin kit from Japan using whatever twins you want but the arivial time is unknown.
The 2540R's comes in two housings .64 and .86
Hope this helped you all as much as it helped me, I might post it as a new thread.