If you are rebarreling anyway you may want to consider some other choices that do better in a short tube than the 7mm rem mag. 7mm's off the blown out '06 case do well with short barrels and you can equal the magnum in performance with less powder and recoil and brass is cheaper. Use a little faster powder that gets more burn in the short tube and you get a little more efficient cartridge. The short mag does fairly well in a short tube with a little faster powder. How much velocity you will lose is dependent on which bullet and powder you use. Heavy long range bullets need a lot of slow burning powder typically which need barrel length to burn or you watch your powder burn in a huge muzzle flash out the barrel which does no good. I have a 7mm JRS and the 7mm gibbs off blown out '06 cases and both will equal the 7 rem mag in a 24" tube with a smaller dose of faster powder.
Predictions are difficult, especially when they involve the future