I would have problems with a plane that didn't get noticeably more lift for a 6 mph speed increase.
Check around, there are 10-turn and IIRC 20-turn pots in various ranges.
Less fine-adjusting but
oh so desirable, check the unit price:
http://www.futurlec.com/PotSliding.shtml
There is a shipping charge but that can be as low as $4 to $5.
http://www.futurlec.com/Delivery.shtml
The package comes from Hong Kong so expect about 2-3 weeks wait.
Something else you could do is put a 10k or smaller pot in series with a 100k pot (or a 1k and 10k) and have both coarse and fine adjustment knobs on the same circuit.