I have roughly the same computer but using a 550ti instead and I've yet to drop below 40fps in a 25m even with all settings maxed.
In either case blizz fails and so does the op's suggestion as the only way to encourage 25m raiding without diminishing 10m is by by removing the shared loot lockout.
Thus the question would no longer be about choosing 10 or 25 it would instead be about choosing if you want to do both.
Even in the past 10m guilds would get together to run 25m content for the additional chance at loot and the last 5 spots were normally filled by people who most likely would have been benched as reserves in 10m