I suppose it depends on what you consider long range. Getting that extra 300 or so fps out of it will shave a couple inches off the 300 yard drop, and about 4 off the 500 yard (10 mph) wind drift. That would take the drift from 27 inches to 22 ish. That isn't going to help much.
A 200 grain .308 bullet at magnum speeds will cut that 500 yard drift in half, and arrive there with 1000 pounds more energy. Id think about that a bit before you get rid of your .300.
The .35 Whelen is at its best as a moderate range hammer. I'd be interested in whether the new load crushes deer or just kills them.