I don't feel like my .951s are narrower than my RC10s. I had to punch out my RC10s for extra width. I haven't had to do that with the .951s. However I'm running the stock liner, molded, in the .951s which is considerably thinner than the Intuition Power Wraps I was using in the RC10s.
I'm loving the .951s, however if UPZs are working perfectly well for you then I'd say there isn't an urgent need to switch. The big difference for me is that I can leave my .951s buckled all day. I was unbuckling the UPZs every chairlift ride. Usually just the lower 3, but sometimes all 5. Otherwise the performance of my .951 World Cups is similar to my UPZ RC10s with stiffer tongues. Haven't had them in sub-zero (F) temps yet so I haven't felt if the flex changes down there. IIRC the UPZs did stiffen up.