Thats a great idea! Do this^^^^
We've had 6.5mm versions of the Ultras for about four and a half years at this point. We sell plenty, but the .30's are definitely more popular. Most of the 6.5mm buyers have multiple cans already and want something to dedicate to their 6.5mm and 6mm rifles.
What you get with a dedicated 6.5mm stack (not just endcap) in the Ultra series is a 2-4 dB improvement, which is about the same difference as going from an Ultra 9 to and Ultra 7. So you put it one way, you can shoot a 6.5 Ultra 7 on your .260 and it will sound almost identical to a .30 cal Ultra 9 on the same rifle.
If someone has multiple cans and a lot of money, sure, buy a 6.5 can. For the other 99.8% of can purchasers, they are a stupid investment.
2-4 decibels is not worth worrying about. Even with a 6.5 suppressor you are still not going to be able to shoot a match without some kind of ear pro, and foam plugs in the ear (but only barely into the canal) knock off way more than 2-4 decibels.
Second, if you are going to shoot with dirty can on your 6.5, you will probably be more accurate with a 30-cal can, because even as the baffles holes get smaller with carbon buildup they will still be large enough to not affect the bullet as much.
The baffle holes do affect the bullet. With my 13 oz. 30-cal can on, my .204 shoots 3/4 MOA
higher than with my 9 oz. .223 suppressor on. You would think the added weight would make it shoot lower, but that is not the case.
As with the 6.5 can you described, my .204 is just as quiet with my 9" 30-cal can on as with my 7" 223 suppressor on. But even my 30-cal in its 7" configuration is
sooooooooooo much better than unsupressed that it really does not matter to me which is on there, noise-wise. The ability to easily clean my baffles and the option of shooting my 30-cal in either a 7" configuration or a 9" is vastly more important that which one knocks off 2-4 decibels more than another.