Having owned an HD-750, an X7, and having tested an X70 (rs55) on my Carada BW real gain 1.1 88" diag screen (16/9) about 3.4m from the screen, I agree with Jason.
The rs55 doesn't bring much in 2D for our screen size. The effect of the e-shift is not visible in a good way IMHO below 100-110", and then only if you sit quite close to the screen, say 1.5SW or closer.
I tested the rs55 and rs45 in a setup very similar to yours, and went for the rs45...
If I had an rs25 and no interest in 3D, I'd definitely keep it, especially if it had good convergence and uniformity, as it's always a lottery to get a good one, irrespective of the price paid.
Also the CMS is the rs25 is, again IMHO, better than the one in the rs55, in case that matters for you.
The main improvement you could expect in 2D would be with motion, and maybe sharpness, but that would not justify the price difference (IMHO) as it is not night and day, just slightly better.