I had a 12 bore double rifle with approximately 1:54 twist. Rather fast, but it shot round balls just fine. From what I've read, this would seem to be the faster limit for twist.

Here's the rifle I'm referring to. A very classic English double of the 1880's:

http://forums.nitroexpress.com/showflat.php?Cat=0&Number=53491&an=0&page=3#Post53491

I believe I would encourage something a bit faster than 1:100 to have the option of shooting conicals.

Curl