Absolutely for sure that 2 channel integrated combined with an AVR will sound better for 2 channels, no question.
An AVR is basically doing the job of 2.5-3 stereo amps (7 channel AVR vs 2 channel stereo amp) and then combine that with the cost of all the licences (DTS, DD, etc) and add a DAC to the question and you can see that a significantly cheaper 2 channel amp just doing 2 channels, with no decoding, no dac, etc will massively outperform it.
The issue is then you have a lopsided front spread, as the centre channel won't be driven the same as the main fronts, how much this will effect you will depend on how loud you play the movies and how good your speakers are etc. You also have the issue of budget, the more money you put into the stereo amp, the less you have for the AVR and therefore the surround sound side will suffer.