I'm surprised Unclenick came back with QL stating ~67,000 PSI. I would have expected much higher. Then again, QL's pressure "guesstimate" is just that and is not measured instrumental data. Sometimes it's close, sometimes it's not.
Personally, I agree with Zeke. I would put a safe maximum velocity with that combo closer to 2,800 fps. The Nosler Partition builds pressure faster than many other types of bullets of the same weight.
A respected gunwriter, John Barsness, after crunching lots of numbers from published loading data has developed a rule for comparing cartridges of the same caliber. The "4-to-1 Rule" says that any difference in case capacity results in 1/4 as much difference in potential velocity, with both cases loaded to the same pressure [Emphasis mine].
Alliant Powder shows 46.0 of RL-22 to give 2,701 fps with a 140g Speer SP in the 260 Remington. Unfortunately, Alliant does not list measured pressure for their loads but SAAMI MAP is at 60,000 PSI for the 260 Rem. Hodgdon's data for the 260 shows measured pressure between 57,000 and 59,000 PSI so I would expect Alliant's to be in-line with that.
Using the "standard" case capacities for both 6.5x55 (57 grains) and 260 Remington (53 grains) that gives the 6.5x55 a 7.5% greater case capacity. Using the 4-to-1 rule that means the 6.5x55 has a 1.875% velocity advantage when loaded to the same pressure (which it normally has a ~9,000 PSI disadvantage compared to the 260 Remington).
That would mean the 6.5x55 could achieve ~2,751 fps with a 140g Speer SP when loaded to the same pressure as the 260 (approximately 60,000 PSI). I would imagine getting up to 2,800 and the higher pressure build up from the Partition would eat up most of the "remaining" pressure and put you at or near enough to 65,000 PSI.
No way would I ever consider 2,950 fps with a 140g Partition a safe load even from a modern 6.5x55...