Most of my shots are 100-150 yd range, but would not worry about a 300-400 yd shot bringing a Whitetail down. Might want to go to a Nosler Partition or something like that is the distances remain that far.
I think that you will find that the 7mm-08 will shoot bullets from 139grn- 150 grn. quite accurately. I have a Tikka T3 that shoots both the 140 grn. game kings and 150 Nosler quite nicely with both Varget and WW760 and these loads are used on deer both Whitetail and Mule/Blacktail deer up to 350 yards with excellent results. I do believe that you will find both the S.D. & BC better with the heavier bullets than with the 120 grn bullet.
Good luck in your testing.
I reload for a hunting friend that shoots a Savage rifle in 7mm-08. His gun loves the Hornady 139 BTSP. I load this bullet over RL-19 powder in RP cases and WLR primers. It produces 2875 ft/sec velocity and it is very accurate out of his guns(half inch 3 shots at 100 yds). This bullet is deadly on deer. He has taken deer from 40 yds out to 310. If your guns shoots this bullet well you will not find a better bullet for deer.
Wendell took his biggest deer last Nov. in Neb at 310 yards. His first shot was too low and just grazed the skin. The next shot took out the heart and the deer dropped in less than 20 yards. It was a perfect shot. We have yet to recover a bullet so the penetration from this bullet is great at the speed of a 7mm-08.
My son just killed a whitetail with his 7mm-08 at 280 yards dropped it in its tracks his first deer, he's 12 years old. with factory remington premier accutip 140 grain ammo but I don't like factory ammo so I will load for his gun I think I can do better.Its a thompson encore with a 24 inch barrel.Some of these loads I will try.It sound like some of you have done well with this gun or caliber.I just think it will do well with a heavyer bullet because he was shooting a little less than 1/2 inch groups at 100yards