Sounds like a good, safe load in your gun. If you lopped off 3" to bring the barrel to 24", you'd likely be right around 3200 fps.
I have a pound of H5010 but haven't used it yet (it's discontinued and I bought it out of curiosity and don't want to get addicted to it). It is said to be of a burn rate similar to H870 (also discontinued

), and US869 (that I have and have used quite a bit).
I don't own a 264 Win Mag, but do have a 6.5/300 Win Mag (300 Win Mag necked down to 6.5mm). It holds about 10 grs more powder than the 264 Win Mag. Mine happens to have a 24" barrel (for now), and with a 140gr Hornady, and the bullet 0.020" off the lands (OAL 3.320"), 84.0 grs of US869 (a full case) gives me 3320 fps (about what you got with the longer barrel - makes sense).
I thought about lengthening the throat to allow an OAL of 3.500" and another 3-4 grs of powder, but the barrel will be replaced so I can't be bothered. A simple alternative to lengthening the throat is to use a "two diameter" bullet (one that has a very long, ogive). The Hornady 160gr is such a bullet, and I can seat it to 3.500" and with US869 approach 3200 fps! Remington sells a two-diameter 140gr that will allow me the same 3.500" OAL, and I have some, I just haven't tried it yet. Neither bullet is a "Match" bullet, but at least you can get a sense of the MV potential.