It's reasonably flat all right (a lot of it's floodplain) but VERY wiggly
I actually went along the Settlers Rd to Wisemans Ferry via St Albans (travelling the other way) when I went down to the Vineyard run earlier in the year. 35km of dirt which google maps estimates takes 1 1/2 hours gives you some clue. Certainly didn't take me that long though, but a windy, sandy, potholed road when I went through nonetheless.
I found it interesting from a historical perspective too as I think it's about as close as you get these days to the original convict built main north road out of Sydney built in the early 1800's.
All of that aside, whether it saves you time or not would depend on how quickly your convoy is prepared to travel over a fairly average dirt road.
Definitely another interesting drive though, and one that most people (even the Sydneysiders) wouldn't normally take.
Newie
PS. Think of me as you go through Paynes Crossing north of Wollombi. It was named for my Great, great, great grandfather who had the first pub there in the 1840's (still there if you know where to look)