How do you think it will sound

Grrrreat ……seriously I think you’ll be very pleased.

Not familiar with your speakers but don't sweat the 5.1. To me it’s like the difference between (quality of electronics parts used by various manuf. taken into account).
Composite to S-V ( a larger improvement)
and S-V to Component. (less drama to the improvement yet DOES make a noted difference)

Or like putting a progressive DVD player on largish display (practically required) or adding one to a smaller display (not so noticeable improvement).

I feel the speaker (count) is not as enormous an improvement to your SS as it is when you revamp.an old PL receiver and upgrade to something like the 950. I just personally perceive it as a smaller net gain when running 6.1 or 7.1 (over 5.1).

I have however a VERY small space where speaker distance is too tight (overlap) . I suspect someone with a cavern for a HT room , would realize a greater gain from 6.1 or 7.1. (just more seamless surround).
The 6th and 7th do add to enjoyment, - but this can also be recording (material) dependent.

By the way Welcome to Outlaw!