From what I have heard, it's just the opposite. The way I understand it is the Kevlars flex very well.
They flex differently to most tires, the outer section of the sidewall is more flexible than the inner section, so you get a lot of flex where you want and not so much flex where you don't.
To get the best flex out of the MT/R's with Kevlar you need to get in to single digit pressures in my view. Due to the inner section of the sidewall being less flexible than the outer section you are less likely to lose a bead at low pressures than most other tires.
KM2's only need to go to around 10 psi to get a lot of flex out of them, but as already mentioned this is due their sidewalls not been that strong, although still stronger than the old KM's.
I ran KM2's and didn't really have anything bad to say about them, they gripped great on just about everything, flexed great, and had very good highway manners.
I think the MT/R with Kevlar is even better, although it isn't a night and day difference. The MT/R's have exceptional grip on rock even when it's wet. They are as quiet as KM2's on the highway, if not quieter. However due to the large voids you can feel them a little more than KM2's.
Either tire will perform exceptionally well both on and off road, but I'd give the edge to the MT/R.