I couldn't tell the difference between M+s in 28 and 32 at all. My times were exactly the same they felt the same.
I think tyre size can be a psychological thing. A jump from 32c to 42c tyres on my Long Haul Trucker made chuff all difference to my average speeds into work.
Tyres I've used in larger sizes;
Schwalbe City Jet (32c) - Decent tyre, although they went on another bike after a few rides.
Vredestein Perfect Max (32c) - Somewhat heavier, I think, and rather poor grip in the wet.
Continental Comfort Contact (42c) - My favourite of the big tyres I've tried. Roll nice, grip well, and hard wearing enough (in that the number of unplanned deflations was manageable, and a worthwhile trade off for the loveliness of the ride).
Continental Touring Plus (42c) - Ok, but very sensitive to being at the wrong pressure (a drop of 10psi has them feeling all wrong). Good handling & grip, slightly heavy feel.