I just played a game of 4/3 vs 5/2 where my opponent got Witch first turn. I lost, but it was surprisingly close.
I've won more than a few Mountebank games on 4/3 vs 5/2. Lost more, of course, but I think a lot of players aren't very good at Mountebank games anyway, so I often just go silver/silver, and hope that I can outplay them in the long run. A lot of R25- players don't have much of a feel for tempo, so will often buy green cards too late in a Mountebank game, I find.