For a bathroom of your size, a 110 cfm fan should be sufficient, providing it's actually exhausting 70 to 80 cfm. A part of your problem could be that the flex ducting is causing an increase in static pressure, which reduces the ability of the fan to move air. Changing the flex to smooth metal ducting might improve the fan's performance. You don't mention the type of roofjack or termination you have on the roof. Does it allow the exhaust stream to exit unimpeded? Does it have a back-draft flap? If so, is the flap opening properly? If it's stuck or doesn't open properly, that could be the source of your problem. Barring that, is the roofjack sized properly? If it's too small it could be constricting the air flow, which also increases static pressure, further reducing the efficiency of the fan. If you want to read a bit more about properly sizing a bathroom exhaust system, I recommend the Fantech website. It contains a great deal of good information for properly sizing and installing an effective bath exhaust system (be sure to check out the downloadable PDF files). The information you'll find there would apply in any setting, regardless of the exhaust fan brand you're considering or have installed. That being said, I have Fantech bath exhaust systems in all of my bathrooms. Not only do they operate in almost total silence, they are extremely effective. I never have a build-up of steam in my bathrooms, regardless of how hot or how long showers or baths go on. I haven't had a foggy bathroom mirror or condensation on windows in years. Here is a link that might be useful: Fantech Bathroom Ventilation

Thanks--your timing is impeccable since we still haven't gotten around to this and I was just working on figuring out exactly which fan to get! We've decided to stick with Panasonic in spite of our bad experience because it is the easiest to fit into the space (same size) and everyone else seems to have good experiences with them, so hopefully ours was just a lemon. Do you have recommendations of specific Panasonics other than the Whisper Fit Lite, though? Our current dead fan is theoretically more than powerful enough for the space (twice what we need based on the linked site) and as far as we know the ducting is good to go (was put in when the fan was four years ago) but we never got very good venting from the first fan. Possibly whatever caused it to die was also he cause of the earlier issues, though.

If you go up, it needs to continue on through the roof with the minimum 6" vent. 8" would be better as a 6" duct can't really do much past 500 CFM successfully. If you can't do another roof penetration (why not???) then up and over would be the best choice. I'd upsize the CFM and ductwork to 10" if you are going to do that.

AhhHaa,if the jack was desighned for rubber shield,that might fit better than conventional metal collar. At any rate,the space between pipe and jack is normal because (A) The jack also work's for double wall pipe from gas water heater which is a little fatter and (B) The space alow's air space when single wall pipe is hot.