Going beyond a 5.1-speaker set-up has tangible benefits. How much of a difference you'll hear when going from 5.1 to 7.1 will depend on a lot of factors: room, speaker placement, processing, etc. But even with simple EX/ES decoding, I think it's worth going beyond a 5.1 speaker layout for three reasons: imaging, envelopment and stability. Follow:

The distinct side vs rear imaging you'll get using 4 surround speakers will be difficult, if not impossible, to achieve using only 2 surround speakers. No magic involved: sounds from the sides will actually come from speakers located there and sounds from the rear will come from speakers behind you. Also, 4 speakers allow for much more precise localization in the surround field, while the individual speakers themselves are less noticeable than if you were only using 2.

All other things being equal, you'll definitely get better envelopment using 4 speakers in a smooth 180 degree arc in the rear hemisphere compared to using just 2 surround speakers. With twice the number of speakers firing at you, a diffuse and ambient surroundfield can be achieved using even direct-firing monopoles. I really appreciate this with music in surround. Plus, level controls allow listeners to weight the ambience more to the sides or behind, depending on personal preference.

Also keep in mind that more 'hard' sources means less reliance on phantom imaging. No matter where you are sitting on your couch, sounds intended to come from behind you will always appear to come from behind you; not some side-ish, rear-ish location. Fancy psychoacoustic processing? Nope. Just a pair of speakers physically located behind you (makes it hard for those sounds to come from any other direction). Stable imaging like that, either to your sides or behind you, is really not possible with only 2 surround speakers. Absolutely yes. Start with a 5.1 set-up now; later (whenever you're comfortable) get a couple more surround speakers and a stereo amp or two more M200s.

Best,
Sanjay