@ 11:36 pm (GMT)
Magnus VassbotnHi Alvaro.
First, I haven't read the whole thread here, so maybe I'm just saying things that have already been mentioned here, and maybe I've missed some points about the stoutness of the TGK vs similar bullets. This reply is just my take on your question of bullet selection for your criteria.
I haven't tried the TGK (or TMK), but I have shot a lot of various medium game with similar bullets (eld-x, a-max, interlock etc), and would not choose any of those for good penetration at close range from a high velocity small caliber rifle at tough game from all angles. One example with the eld-x, was a red deer yearling I had to give a follow up from behind at around 220 meters with a 6,5x55. The bullet struck square in the finest part of the rump, smashing the bone and the whole rear leg, anchoring the animal for a killing shot. But even at a moderate impact velocity of 2400 fps, the bullet failed to penetrate further than just through the thigh (lots of muscle and bone/ resistance). My buddies and I have done similar stuff with the Accubond 140 and Partition 140, and they generally reach vitals with such shots.
In the case of the 264 win mag and your criteria, it really screams Partition 140 or Barnes TTSX 120. Both will have a pbr of just under 300 meters with a max 3 inch high/ low, and the Barnes will still have good killing effect at just about that range (Partition way further than that). I haven't tried the Partition at the close range impact velocities you will get, so others can say more there (or check the 264 win mag article). I have however shot quite a bit of game with high velocity light weight monos (3200-3400 at the muzzle), doing 0-300 meters pbr hunting, just like your criteria, and that has generally worked well. Close range penetration is flawless, and max range wounding is still good. In my experience, this is the one field where monos really shine - pbr hunting with high velocity loads.
As for bc (TGK vs Partition/ TTSX), a few points in bc up or down is not really noticeable with regards to trajectory or wind drift inside 300 meters anyway. The only practical difference will be in impact velocity, but as long as the above mentioned bullets have enough for 300 meters, it is better to put emphasis at the close range/ all angles performance. At least I would, considering it is for driven hunts.
Good luck and merry christmas.