Part of the difference tween the Colt carbine and the 280 Imp is the drop in the BC stock causes a lot of the recoil to go up, not just back, I had to really hold the barrel down when I was shooting it, not an issue with the extra barrel weight of a 45-70 BC with even hotter loads, coarse I have weight in the 45-70 and 45-120 stocks too!!
I don't know what it is about the 280 Imp over the 280, there was a thread on Improved cartridges at PredatorMasters last year, the 280 Imp got some complaints about recoil, that was before I had shot mine, I have to agree with them, it does have noticeably more recoil than the 280.
Tim