I'm using sync for efficiency. Looking at the MCP1642, it works at 2MHz and a 4.7uH inductor. I reckon both of its fet's are driven using an emitter follower, the Rds for both is relatively high for low voltage fets so probably the gate capacitance is small on both (I've determined from viewing many datasheets there is a direct link).
As I could only improve on the MCP1642 by using an N fet top-side and a boosted drive voltage, and because all of this adds to the footprint space, I've decided to abort using the PIC here and go with the MCP1642.