I took my 20lb 338 lapua improved for a 42 mile horse back ride into the Thorofare. I was glad i had my long range tactical rifle with me. I ended up killing a very nice 6x6 at 1103 yards (one shot kill). One thing you might want to do is to have a custom scabbard built for your rifle. Many of the guides dont have scabbards large enough to handel a large rifle with a big scope. My rifle has a McMillan A-5 stock and a NSX night force scope.
I went back to the Thorofare the next year and I took my Kimber Montana chambered in 300 wsm
this is a 6lb rifle with a scope on it. My hunting partner Derek and I hunted very very hard. The last morning we found a 5x5 bull elk at 574 yards. Derek didnt feel confortable shooting that far so I decided to take the first shot. I hit the elk and it just stood there so Derek and I decided to keep shooting until the elk was down. Between the two of us 16 shots later the elk was down. To make a long story short my 6 lb light rifle had so much muzzle jump that it actually came up off my back pack and cut my forehead. I was completely embarrased because I do a lot of rifle shooting and could not believe that me. When we recovered the elk it had 9 bullet holes in it. I could not believe it did not go down sooner we had 5 great kill shots on the animal.
Buy a large scabbard take your 15lb tactical rifle with you and be happy. Remember you are hunting from a horse and the horse will carry your rifle most of the way for you.