Depends totally upon the circuit. If it's tight and you seldom exceed 100mph ish, I'd stick it on max down force. Somewhere like Bedford or Silverstone GP where (depending upon your car) you'll be seeing 140mph+, you'd do best to have it mid way somewhere, IMHO. I do think it has an effect although not drastic.
Mark's got a 33 so he hasn't got any undertrays unless my car has it's nicked by somebody....? I realise 34 Vspecs have but 33's don't. Anyways, can't see they do much, not having them hasn't made much difference to me when up against the odd 34.
PLEASE don't come on here mentioning that picture. I've always realised that I'm not photogenic but I really was caught in a bad moment there! Did your missus have her specs on or was she pi55ed?