Forty Four,
If you can pick up the latest Rifle magazine (Jan 2002) Brian Pierce has an excellent article about pistol cartridges in lever guns. It'll answer all your questions. Beware though, after you read this, you'll be heading to the gun store. I'm speaking from experience.
To summarize a few key points, the majority of the velocity is generated in the first 16 inches. Each inch thereafter is worth about 15 fps with pistol cartridges, a negligable amount in my opinion. The 44 mag carbine with a 16 inch barrel will push a 300 grain bullet to 1600 fps + (with H110/296) and be well within pressure specs and case life should be excellent. In the Rifle article, a factory 300gr load produced almost 1700 fps with a 20 inch barrel. I myself went with the .45 Colt because of the same-velocity-heavier-bullet-lower-pressure thing, but mostly because I already reload for my Ruger Bisley in the same caliber.
Good luck!