Pepin,

the downhaul technique have been tried and left again on the bigger boats. It was very "in vouge" for a while and the french Tornado team used in the games at Athens. I dont know of any other teams using it there though (after 2004 I lost track).. Trimming the downhaul instead of the sheet makes it very easy to end up with a main much flatter than desireable. To release downhaul properly to re-shape the sail you also need to relase a bit of mainsheet, so I think this will only work in special cases.
About cleating and then driving.. What does Ashby and the likes do on their solo boats? I think they are sheeting actively and helming straight or on waves. I have not checked so I could be wrong.

Mark,
you find it impossible to oversheet your main with a 1:8 mainsheet? Becouse it is too hard to sheet in more? If that is the matter it sounds like you could need more purchase, less friction in your system, better technique or more strength Pick any one you want to improve on
(or release some downhaul to oversheet and go even slower.. )

I dont think how you hold your sheet is going to make a large difference. Larger diameter and softer sheet at the working end is good. Strong grip with good gloves is good. Splice in a 4-6mm dynema line for the parts not running through the ratchet in the system or move to a in-boom cascade system for less friction.