"...ordered some...and got sent..." One of the reasons to buy ammo and components locally.
Superformance is the current name for the old 'Light Magnum' line. Basically max loads from a factory.
Like std7mag says, the SST(Super Shock Tip. That being marketing nonsense). is a lead cored bullet with a hard polymer tip. It's Hornady's answer to the BT, Accutip and the original Sabre Tip from CIL. The GMX is their answer to the Barnes solid copper bullet.
150 grain .308 show the GMX is running a whole 60 FPS slower than the SST. Slightly different ballistics too. So if you opt to keep 'em you'll have to sight in again. And at 500 , the only thing that matters is the remaining energy(59 ft-lbs. less for the GMX) and if you can hit the proverbial pie plate at that distance.
The "weather" will be decided for you. So will being able to return 'em. Like Scorch
says, a lot of shops do not do returns on ammo. Find out with a phone call or e-mail.