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.
*Notes
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