I also just got a new .280 recently, and I'm in the same boat. I see that this thread is from a couple of years ago, and I wonder how you made out. The guy I bought the rifle from has one just like it, and he is using H-4350 and the 145-grain Barnes LRX. ( Both are Dakota Model 10's, with 24" barrels.) He's getting 3050 fps muzzle velocity, and excellent accuracy. Since I have three 8-pounders of that powder, I'm starting with that. I also have plenty of H-4831, so that will be my next try if the 4350 doesn't work out to my satisfaction. Between the two powders, I think I'll find a suitable load for the 140-grain GMX I'm working with.
I've shot a bunch of animals with the 165-grain GMX in my 30-06 at about the same velocity we're talking about ( the Superformance factory load shoots well in my rifle ) and I like this bullet. If I don't get the velocity and/or accuracy I'm looking for with this one, the Barnes bullet will be the next one to try. I have no issues with conventional bullets, and I'm a BIG fan of the Nosler Partition. I have come to prefer the all-copper bullets these last several years, since they don't make such a mess near the entrance wound. ( I'm a PETA kind of guy - People Eating Tasty Animals.) I used to hear a lot about guys who want an exit wound to give them a blood trail, but copper bullets don't make big exit wounds where that little rivet comes out. I haven't needed any blood trails, since they mostly just fall down right where they are where I shoot them. I think that the slightly lighter 7mm bullet will probably work about the same on deer & elk as the 165-grain 30-caliber bullets I have been working. I'll let you know if I find out otherwise.
Keep us all posted on how it goes with your loading & shooting, Sir. Thanks.
Nick