This is a very complex discussion and how you go depends on how serious you are both from a sonics, and from a budget point of view.
The guy you spoke to is partially correct I expect. If your processor does indeed bypass all of the digital when in 2 channel mode - this may (or may not) help. Your power amp will indeed optimize on 2 channels when you're only driving 2, because the power supply is only supplying those 2. Your amp is a pretty good amp too; however, I doubt your amp compares favorably to a quality 2 channel only amp, and likewise the 2 channel performance of your processor probably compares poorly to a quality 2 channel pure analog preamp. What you need to consider is whether you want to invest in a separate 2 channel system, and connect it in "bypass mode" with your front speakers.
Bypass mode worksa as follows: You buy and install a high quality pure 2 channel preamp and a hi quality 2 channel power amp (or better still IMHO: 2 mono block amps) and connect then to drive your main speakers.When you are listening to 2 channel music exclusively, you are using this system, and you processor and 200.7 amp is turned off.
To listen to theater, the "main speaker" outputs from your processor are connect to your stereo preamp "passthru" inputs, which pass the front channel signal through to your main speakers. This can be an expensive way to go, but it is hands down, and far away better for 2 channel music performance, and no worse for theater performance than whayt you have.
Some stereo preamps have built-in passthrough channels, but even if the one you want doesn't it's possible to set it up to work this way. The nice thing about preamps with passthrough built in is many of then need not be powered on for HT mode.
Finally - could you improve your performance by simply adding a dedicated 2 channel amp and not adding an analog stereo preamp? yes you can - with the right stereo amp (or pair of monoblocks). This won't get you all the way there though . . .