You're right, Don- the Kawken has a descent shape, but the butt is the shoulder-joint killer. It sits on the rotator cuff adn tears things up with heavy loads.
A .54 would be OK. I had an 11 1/2 pound (34" X 1 1/8" .58 KHawken) which needed 140gr. 2f to shoot reasonably well at longer ranges - ie: 100 yards and beyond. In those days, I didn't shoot at close range like many people today. That butt plate beat me up badly and I sold it. Always lamented that I'd sold one of Taylor's rifles, but it had to go(at that time).
I agree with Dan - .54 is about tops in that style. Here's one shooting 95gr. 2F as a target load. My late hunting buddy, Brad.
His Daughter, Jody shooting 'Mom's' full stocked Hawken .58 with 90gr. 2F - she does great with it, but, it certainly kicks a bit more than 'Dad's' .54 - notice the muzzle rise in comparrison. The only difference is about a 50gr. heavier ball. The Full stock weighs more than Brad's 1/2 stock as well. Jody's hold will contribute a bit to elevation of the muzzle. Note sparks from the muzzle of the .58. No - it isn't powder burning out side the bore. It's red hot fouling chunks.