If you need P45+ sample files for long exposure let me know. They are very high quality at far beyond 4 minutes. The dark frame subtraction is non-optional, but the results are fantastic.
I'll have to take you up on that offer now - could you shoot a P45+ RAW from a 1 hour exposure at ISO50 to me? I'm this close to taking the plunge, though I am a little bit worried about how well 3rd party backs fit on the H2... someone suggested that if fitment was just a couple of microns 'off', I could kiss focus goodbye.