No experience with the 338 Fed, but I have had a BLR in 358 Winchester now for 11-12 years and have always been impressed with it. I monkeyed around with some 200's and 250's early on, but just settled on a hunting load of W748, WW cases, and a 225 Nosler Partition or Sierra 225 at 2450/2500. They both shoot identically and have been shot to 400 yards. I have a plain old Burris 2x7 on my rifle and it's really no big trick to get them out to 400 yards. I can't see one being any better than the other, so it would just come down to what you like. I like the .358's so I shoot a bunch of them.. Been using the 180 TTSX in the 350 Rem Mag as well which might be another great one in the 358. I'd think a rifle set up like you stated would be a killer little rifle. I have no idea how much better it would be than your current 308 with 180's would be, but I know the 225 Partition and Sierra are pretty danged awesome on deer which is all I have taken. They put large holes through em, and I have never caught one of them in any deer, even with off, angling shots. It's a fun, easy shooting cartridge.

Semper Fi