I have a 22" 6TCU barrel that I bought back in the spring, I have not shot it a lot, but 70 gr. Speer TNT will do 3075 fps, 80 gr. Remington HP will do 2900 fps with no problem. I have not tried for max speed, these shoot and extract easily.
The Hodgdon #27 manual has loads for the 6TCU, 6x45, and 6x47. They are all pretty close. It would not be a lot of difference between all of them if they were loaded to the same pressure.