As you have said, this is a difficult question. It's hard to get enough data to really say. But here are a couple of indicators:
One of the Scandinavian countries made an attempt to correlate the rapidity of the kill with the cartridge used. The Swedish 6.5x55 did quite well on alg, and I think you can depend on the 260 being indistinguishable from that.
My own opinion is that the 260 is closer to the 30-06 in killing power than many people suspect. It will reliably drill a 14-16" hole, and the chances of that transecting a vital spot are quite good.
My main go-to big game gun is a 7x57 loaded to modern pressures and using heavy for caliber bullets. I think it is quite adequate for anything I care to hunt, since I'm not interested in the large bears. The 260 is not really that much different from the 7x57.
As you pointed out, one key is shot placement. Another is bullet construction. Standard bullets generally need to impact between 2100 and 2800 FPS, and Partitions need to impact somewhere above about 1800 FPS. If you are using the appropriate bullet impacting at the appropriate speed and putting it in the right place, then I think that the fact you are using a 6.5mm 140 bullet instead of 30 caliber 180 grain bullet is less important.
