If someone with a 50# 4bar fuel setup, switched to 72# inj, would there be any use in an adjustable FPR? what would it benefit, if the 72's run at stock fuel pressure (3 bar)??

Well, unless you are planning to push the envelope with the 72 lb injectors, there really wouldn't be a need for it. You an use it to help fine tune or to richen the car up top while in full boost.

So it's really useless in say a 400whp application, since there is still plenty of room w/72s. would the gain from "fine tuning" be worth it, or would it just be better to leave it the way JWT made it?

The MSD 50 lb injectors w/ 4 Bar fuel program can support 420 whp. If you are only looking at making 400 whp, then the 4 bar is perfect. You woudl have to play with fuel pressures on th edyno to notice any gains.

I know that 50lb, 4bar will support 400whp, but I like the simplicity of 72's. also, they offer more headroom, if you ever deceided you needed more power.