You could try increasing gamma and gain in the color corrector FX it will help but will also reveal the noise in the blacks. That’s a pretty bright background behind, so that was obviously the original problem.
I’ll try render one of my 20 min projects in 1080i for you see what happens, not had a problem i can remember when i have done this previously. Get back here soon.
I don’t have any problems rendering from 9c with 1080 avchd from either Panasonic HMC152 or Canon HF10. My videos are usually 20 mins long and exported 720p m2t. Audio remains in syc, PC is only a Core2duo XP,4gb ram, so not everyone is having problems. Same on my Window 7 laptop, no problems.