Well for the benefit of anyone who cares (and to save GMballistic some time/expense of experimenting ), I've been playing again.
Firstly, a week or two after the first post I washed the car again and the sheeting behaviour has changed - namely the sections topped with C2 are now sheeting at the same rate as each other again with FK sheeting slightly slower. It dawned on me that the only change in my wash routine was the shampoo used. First time round I used Chemical Guys Mr Pink, which I've since realised does contain gloss enhancers so this could well have been affecting the water behaviour. I've since used G|Technique G Wash and Car Chem Tailor Made (which don't leave anything behind) and the sheeting's much more uniform between the panels topped with C2 and those now topped with FK.
Secondly, I've also topped up several panels using diluted C2 (roughly 3:2 - I simply topped up the remaining ~20ml in the Waxybox sample with de-ionised water to make ~50ml) and using it as a QD. That was 5 weeks ago now and two coats give a much slicker finish than just the one, it's also easier to apply the second coat too (not that applying the first is not easy of course) and those panels with two coats of C2 on are staying cleaner and sheet water quicker than those with just the one coat. I've had to resort to doing productive things on a weekend because the car just doesn't need cleaning as much and even when it does, it doesn't take as long any more.
Thirdly, I've been playing with waxes on top of C2 on the roof. I tapped off several sections so each section is separated by bare C2 to make it easier to see the difference in water behaviour and finish. I applied Angelwax Desirable, Poorboys Natty's white, FK1000P, and three Waxybox waxes - May's #008, the Anniversary wax and the Halloween wax. The latter three lasted 2, 3 and 3 weeks (give or take a couple of days) respectively before disappearing to nothingness, Desirable and the Natty's are just about hanging in there and FK1000P is still going strong 5 weeks later.
Finally, I accidentally left an open pot of Waxybox wax on top of a radiator - I got distracted by something shiny. As soon as the heating was switched on, it did a good job of making the whole room smell of the wax... luckily it was one of the nicer scented waxes (it was the winter wax blend, #006, I think in case you're interested), so I left it there. It's still making the room smell very pleasant indeed, weeks after the other waxes have failed on the paintwork.
So to summarise; know your shampoos before blaming the LSP, use 2 coats of C2, C2 plays well with other LSPs and previous Waxybox waxes make better air fresheners than they do waxes.