Flump- do you locate them parallel with the boat or do you angle them so that the front bolt is more outboard than the back bolt? It seems like this would give your foot more resistance when trapping.
I locate them parallel to the hull. A photo of Booth's boat at the Aruba regatta shows his parallel to the hull as well. Windsurfer back foot straps tend to be parallel to the board as well - so I think a parallel position gives enough resistance.
Too much resistance can be risky. I stuffed hard on the wire with some fancy chicken line system I copied from Jamie Livingstone and my legs gave way before the chicken system. I thought I'd broken something and was hobbling for weeks afterwards. I think the foot straps give enough resistance for racing round the cans. For distance racing a chicken line gives that extra security.