Any question answered...

I'm about to order Shimano dual pivot calipers and notice that it says they "Must be used in conjunction with ST-4600, ST-4603, ST-5700, ST-5703, ST-6700 or ST-7900 STIs for correct leverage ratio". Is this true? Or is it the same as the recipe instructions on my Sainsbury chicken, "baste with Sainsburys butter, stuff with your favourite Sainsburys stuffing and place in a pre-heated oven"? That is, bullshit?

They changed the actuation ratio in the latest round of upgrades- (from memory) they reduced the mechanical advantage of the levers and increased it at the caliper.

This means that old style Shimano, or any SRAM or Campag levers when combined with the new calipers are, in Shimano's view, "too strong".

Basically, you'll have more power but less modulation I think.

Basically, yeah - the pull ratio is different. They tend to end up a bit grabbier when used with most other road levers, especially the rear brake. However:

I use 7900 levers with another manufacturors brakes and they're fine, that said.

This way round is usually fine and less noticeable, but you would probably get better performance with the 'correct' matching brakes.

FWIW I run a 6700 front brake with a conventional lever because I like the extra power, especially from the hoods. It's a teeny bit harder to modulate at low speed but it's really not a problem. So it depends on your setup. If you're running a rear brake you might lock-up a bit too easily.