You have quite a generous definition for "largely similar".
According to some Laurie Holland, 6.5x55 has 6.5% more case capacity than Creedmoor:
http://www.targetshooter.co.uk/?p=2100
According to Vihtavuori data (from older print manual, not present in current online manual) 1% increase in case capacity results in 0.7% decrease in MV and 4% decrease in peak pressure. Of course these are approximations etc.
From same source, 1% increase in powder charge results in 0.8% increase in MV and 2% increase in peak pressure.
So using this info w/o further consideration, increasing case capacity 6.5% would lower the peak pressure 26% and MV 4.6%. To regain the MV you'd need to bump up the powder charge 5.7% which would increase the peak pressure 11.4%. You'd end up with same MV and 15% less peak pressure.
You have read that COMPLETELY wrongly and come to conclusions that are 180-deg out. If correct, a .300 Winchester Magnum which has nearly 50% greater case capacity than that of .308 Win would have around 40% lower MVs than the latter. Real life suggests a rather different outcome! Also, please reread your Viht manual - a case capacity increase without any other other change
decreases pressures.
What you're saying only applies if the same bullet and powder grade / charge weight are transferred to the larger capacity case
without any change to the charge to suit the new circumstances. That is to take a 165gn bullet and say 45gn powder from the 308 and transfer them to the 300 Win Mag.
What does happen as case capacity increases is that the cartridge becomes less thermodynamically efficient if everything else is 'equal' - ie less of the powder charge's energy content is converted to useful work in moving the bullet faster. Eventually when the combination reaches a true 'over bore capacity' condition, the extra powder provides no extra velocity at all - hence not many .50BMG/.30 wildcats around, nor even many .300 Weatherby Magnums.
The rule of thumb is that in a given calibre a given % increase in case capacity provides an MV increase of that % divided by four
all other things being equal. The 'other things' are barrel length and characteristics, bullet weight and characteristics, peak chamber pressure. So ... if a fireformed Winchester .300 Win Mag case has 48% greater capacity than the equivalent Lapua .308 Win hull, and the 308 produces 2,800 fps with an XYZ gn weight ABC model bullet from a PQ-inches length barrel, the 48% capacity increase will power that bullet to a 12% higher MV if both are loaded to the 308's 60,000 psi allowed maximum pressure. Or in this example, an extra 336 fps = 3,136 fps. However ......... the .300 Win Mag is rated at 65,000 psi MAP by SAAMI, so it gets a bit higher MVs in practice. This and any other such example also assumes the correct burning rate propellant is used, so the 308's Viht N140 would be replaced by N560, N165, or maybe even N170 to achieve the cartridge's full allowed pressure rating.
When it comes to the 6.5X55mm v modern higher pressure rated cartridges, life becomes complicated because we are trying to compare a larger capacity and powder charge but low pressure model against a same calibre smaller but higher allowed pressure design. Moreover we have four (yes four) different 6.5X55 peak pressures:
US SAAMI .............. 46,000 CUP (Copper Units of Pressure, a now obsolete measurement system propably equal to around 50,000 psi using the modern Piezo crystal transducer or strain gauge methods)
Actual US sporting ammunition loadings ............ under 40,000 psi on the basis of actual MVs produced.
Actual Swedish military loadings ............. ~45,000 psi for early 144gn RNFMJ ammunitions, nearer 50,000 psi for the later 139gn FMJBT 'spitzer' HV round and proof tested to 68,000 psi. Lapua 139gn Scenar Match is loaded in line with the early lower figure because it is likely to be fired in 100 year old M1896 service rifles and similar.
CIP (European regulatory and standards body) for 6.5X55mm SKAN (for modern firearms in good condition) .............. 55,000 psi. Possibly what Norma and Sako load their sporting ammo to, but I can't be sure as I've never tested any.
There is a fifth possible level - handloads using modern Lapua brass in a strong modern rifle and up to 60,000 psi (but at your own risk!). I have had 2,999 fps from the cartridge with a 139gn Lapua Scenar and no primer pocket or extraction issues in a 30-inch barrel F-Class rifle. I wouldn't ever use or recommend this level, and my long-range match loading was 2,815 fps from the 140gn Berger Target LRBT over Viht N165 which is right on Viht's 55,000 psi SKAN loading and likely produces a little less.
That's what the 260 Rem and 6.5mm Creedmoor will give at their high 50,000s approaching 60,000 psi maxima, and will be feasible for the latter if not a bit better when Lapua small primer brass arrives.
