My 6.5mm AX is getting 3300 fps with a 140 gr Partition in a 26" barrel. I have only been able to test one load so far and will do more testing soon but it is a pretty potent round.
Wildcat Bullets offers ULD RBBTs up to 169.5 gr in 6.5mm and pretty much any weight down from that as well. There is also rumored to be an Accubond in the works as well.
A standard 6.5-300 Dakota would be a great choice for a 600 yard whitetail rifle. For all practical purposes identical to my 6.5mm AX. Performance equal to a 6.5 STW but in a much more compact package. This was the design reason for my 6.5 AX.
This is the 264 Win Mag on the left, 6.5mm AX in the middle and 270 AM on the right.
A 270 version with the Wildcat 169.5 gr ULD RBBT would also be quite impressive as already mentioned!! There will also be much heavier 270 Wildcat bullets coming out soon as well.