The Baikals are proofed for loads to 32,000psi, apparently.

At pressures around 28,000psi - the 3 1/4 case will drive

350gr. to 2,021fps,
405gr. to 2,012fps
500gr. to 1,809fps

All with lead bullets in 24" bl. and all just barely over what the same pressures will do in the 2 1/0" case.
This according to Hodgdon's annual manual.

At the same pressures in the 2.1" case, Hodgdon lists

300gr.Jacketed HP at 2,143fps
350gr.jacketed FP at 2,036fps
405gr.jacketed FP at 1,859fps (26,500cup)

Lee's listing for the 2.1" and lead:

405gr. 1,817fps @ 27,400psi
420gr. 1,763fps @ 27,400psi
440gr. 1,652fps @ 27,300psi
460gr. 1,636fps @ 27,100psi
500gr. 1,510fps @ 27,900psi

Lee's jacketed listings for jacketed 2.1" case

300gr. 2,125fps @ 28,700psi
350gr. 1,890fps @ 27,800psi
400gr. 1,859fps @ 26,500psi
500gr. 1,510fps @ 27,900psi

Speer's .45/70 data listed for the lever guns, according to their text says they held pressures to 28,000cup - which in the .45/70 is the same number as PSI.
Their data shows over 2,000fps with 400gr. jacketed.

I really don't think there is a lot of difference in the longer case when dealing with these lower pressures. There is some difference, but not a lot.

I would not do it, personally, due to the taper of the barrels and possible troubles with regulation. Once done, it is done. Reducing loads in the longer case presents a problem with excessive case capacity and having to use case fillers.