I have been on my roof in both wet and dry conditions, both before and after painting with the Rustoleum deck paint, and I see no difference in how slippery the surface is. I have never experienced what I would consider a slippery surface condition on my roof since I painted it. 
My expectation is that a waxed roof, Mop and Glo or otherwise, would tend to be more slick than an unwaxed surface. I have waxed boat decks and have had dramatic changes in how slippery the surface had becomes to the point where I considered the waxed surface dangerous. I have never waxed any of my RVs roofs just for that reason.
I personally chose not to add the Silicate to the mix and from my perspective it is fine although others might chose to do so.  When I do plan on going up and pressure washing I do wear Sperry boat deck shoes for safety as I always did on a wet boat deck. At least the motorhome is a flat level surface that is not pitching or heeling and for that I am thankful. It is really a quick job to clean the roof now, I use a swivel head arrangement with a handle that has a white scrub pad on the bottom that is from the marine industry for scrubbing decks.
The big plus to this approach is cost, relative ease of prep followed by roller application, it should last for years where Mop and Glo would be months at best, and it truly protects the gel coat. I still do not see any downside to this approach but will keep you posted if one comes up.  
Later Ed