I'm also interested in this topic. I specifically got the Blitz SE return flow cooler for my car, as many people have made good power with them. I haven't really gotten the results I was hoping for, and my tuner is convinced it's because of the cooler. The only difference I can think of is that I'm aiming to run fairly high boost 20psi+ which makes me think that perhaps the pressure drop across these return flow coolers get magnified at higher pressures.
Be very interested to see results from Stao's testing on his R34!
Also, anyone have any experience with the Plazmaman entry level cores?