Gordon,
I've driven the Plus 6 and spent some time looking around the Williams example. I think your analysis is spot on: it is a cracking car, other than the seats. The engine and gearbox combination is one of the best available: BMW developed them to work together and they were the best bit of my ownership of a M140i. I don't think BMW offers the engine with a manual box in any vehicle, though I could be wrong.
As others have said, the chassis, with a smaller engine and manual box option, will be a cracker.
Anyway, this Aero Plus 8 owner wouldn't consider changing, mainly for the reason that Elliott states, but also because in the UK the big Plus 8 is really rather rare!
