seems to spark well.
I hunted and shot a houseful of T/C Hawkens year round for 18 years.
Old style locks then upgraded to the new style locks
Old style vent liners then upgraded to the new style vent liners
Accumulated several different calibers and gauges of T/C and GM drop-in barrels.
The new style locks and vent liners made them seem like I was shooting a Marlin .30-30 with:
Goex 4F prime
Goex 3F (or 2F) main
Tom Fuller black English flints
Plus, remember that T/C and GM barrels have a modern version of Nock's Patent Breech so a vent pick / picking the vent is not needed...I never owned one.
Due to variations in the inletting locations from stock to stock, every now and then when I converted a caplock to a flintlock, the location of the vent would be forward or backward of center on the pan and never caused the first problem.
So if it was me, I'd forget about all the vent reworking if it involved redrilling the barrel as I don't believe that's your problem, and it would also void the barrel warranty with GM)..... and try 3 things if you haven't already done so:
1) Your choice of words "SEEMS to be sparking well" caught my eye.
If you're using T/C agate flints, strongly suggest you get rid of them and use Fuller Black English...T/Cs new locks with Fuller flints made showers of sparks.
2) Switch to 4F prime (or Null-B) if you are not already using one of them.
3) And if you haven't been pump flushing the Patent Breech 100% clean after every use.
(or wrapping a cleaning patch around a small diameter brush like a .32cal to slide down into the Patent Breech powder chamber and clean it every time) it could have accumulated a build up on the walls of the powder chamber and flash channel that could be keeping main charge from flowing freely all the way down and around to just inside the vent liner...you should be able to see kernels of powder in the vent hole.
Just my .02 cents on the matter...