According to Winfer, the 461 No.1 using a 480gr bullet and 85grs of black went about 1300fps.
Selous was reportedly a fan of the 540/75 load in the 461 No.1. I have used a similar loading in my 461 but didn't shoot it at anything past 100yds. No idea what the velocity was.
The 461 No.2 with a 570gr bullet using 90grs of powder got to 1350fps.
The express load with a 360gr bullet and 100grs powder went 1700fps.

That sounds pretty respectable to me!

Interestingly, Gibbs often sighted their 461s for both the heavy bullet loading and the express loading. Apparently, both shot well enough with the same twist Metford rifling. Given George Gibbs' history of match shooting and producing top tier match rifles, I wouldn't think they would turn out any rifles which wouldn't perform admirably.