If you want to get a 338 Win Mag go for it. I've used mine for sitka blacktails, caribou, moose, brownies, and goats with no problems. I shoot the 225 grain Barnes X bullet at 2820 fps out of a 26 inch Browning Stainless Stalker. Since you asked about trajectory, here's mine which I carry with me in the field.

100 yards +3.9
200 yards +4.5
300 yards -0.8
400 yards -12.9
500 yards -32.7

These figures are based on an average velocity of 2820 fps with a 10 inch kill zone and a maximum point blank range of 341 yards. This load suits me just fine. Good hunting...

Your reasoning is sound!
If you don't hand load , I would go for the .338. The Weatherby ammo is awfully expensive ! I live in Oregon, and My 270 and .338 combo is about perfect for this location, If I were in Alaska, I would not be comfortable in bear country with a .270, although with a barnes tripple shock you might just stop a pissed griz but I would feel much better with a .338. Your 300 weatherby is a great compromize, but depending on the model and its condition you could probly get enough out of it to get a .338 and a good start on a reloading set up !!! ...tj3006

If you have a .300 Weatherby, you don't need either the .338 Win. or the .270 Win. If you can't kill everything short of Asiatic buffalo, cape buffalo, hippo, elephant, rhino, and maybe Alaskan brown bear with a strong 180 gr. or 200 gr. bullet out of a .300 of some sort, you're not going to kill 'em with anything else

For big game, I'm convinced that I can literally hunt the world with just my .300 Win. Mag. and .416 Rem. Mag. and never look back. I'm pretty-much there right now.

AD

I'm with Magnum hunter, I think the 338 Win is an excellent cartridge and have one, but I also have a 300 Win. and wouldn't want to be without that also. Use the 270 for 130-140 gr. loads, your 300 for 165 or 180 gr. loads, and then the 338 with 225 and 250 gr. loads fits perfectly below the 416.

Of course then you might need something between the 270 and 300 like an '06, then something between the 338 and 416...

Since I have both I will jump in here. I could no longer live without either one. Keep the 300 Wby. and get you a 338 Win. I have shot caribou, elk, moose, bear, whitetails with the 338, it is a necessity for anyone's battery (or the 340WBy.) I coveted a 300 WBy for years until I found a Classicmark several years ago. I shot mule deer and whitetails with it then took it to Zimbabwe and used it on plains game. It was the perfect compliment to my 375 H&H. I personally think the 300 kicks harder than my 338 but thats for everyone to decide for themself. Keep the 300, but get a 338 too. Thats my advice.
Doug