150gr NP will work fine for your elk hunt. My hunting partner uses speer 160 gr Grand Slams in his 280 rem which work very well on elk, moose or deer. Nitrex ammo use to load this for the 280 and it was reasonable to boot. You may try locating some of the 160 gr grandslams by Nitrex. The TBBC loaded by Federal would be another solid choice. Personally I think the 140 gr Failsafes should also get the job done. I have used them on both moose and elk in a 270 win with good results, I just limited my shots to ensure higher % outcomes.

Have you tried conley ammo out of MS http://www.cpcartridge.com , I have just purchased Barnes X and Nosler partition ammo from them in 160 grains and they shoot great the ammo is more accurate than I am, but they also have a big selection of 175 Grain to, Nosler, Swift , Barnes , and more

Winchester loads the 140 gr Failsafe in factory form. It is the only factory load I shoot in any of my .280 REM rifles. It will kill any elk ever made no problem. The failsafe is an excellent bullet and due to its design you can use a lighter weight bullet that will expand properly and easily penetrate all the way through. This is a do it all load for the .280 except for the larger bears, if you are shooting factory fodder. Hornady light mag is only made with the 139 gr loading.

Personally I shoot the 160 gr Nosler Accubond at everything I shoot with the .280 in my handloads.