Many rifles with 1:48 twist or faster are very good at shooting conicals. Flintlocks are no different although I kind of agree with RollingB that a well placed roundball is usually all that is necessary.
They had a heck of a sale on powerbelts this month. So I bought four packs of them for the .54 caliber. Today I took out the Lyman Trade Rifle Flintlock which has a 1:48 twist. I was shooting 405 grain copper plated HP Powerbelts with 85 grains of Goex 2f. Shot #8 was the last one of the day and it was a delayed fire. I did find that the heavy powerbelts shot much lower then the roundballs...
I think with some work I could make this Flintlock shoot the powerbelts real well. I also have some other conicals to try. But for the most part I really like shooting the roundballs...