Shimano have changed their 'road' brake cable pull. The new brake cable pull 'New Super SLR' (NSSLR) is significantly longer than the old one and the levers have a lower mechanical advantage (MA). With (I think) just one exception, if you are using STIs with a model launch date 2008 or later, they are NSSLR type, and should be matched with NSSLR type calipers.
It isn't a small difference BTW; NSSLR calipers have an MA of about 1.75 (or more if the brake blocks are high up in the slots) and older DPs have an MA of ~1.25 to 1.5 or so.
Shimano don't make a NSSLR type longer reach caliper, so the BR451/BR650 etc model calipers (which work really well with 1992-2008 STIs BTW) limp on in their model range with (as pointed out above) a dotted line in the compatibility charts, indicating that you get 'reduced power'.
In fact it is worse than that; you would presumably only buy longer reach calipers if you actually need them, which means that the brake blocks are likely to be in the lower half of the adjustment slots anyway, thus further weakening the brake power....
IIRC there is a Tektro model of brake that has a longer reach and is NSSLR compatible; it is identified by Tektro as being 'new type pull ratio' or something. If in doubt you can look at the layout of a DP caliper and see if it is similar to a known NSSLR type one or not; NSSLR calipers are laid out quite differently from older type DPs.
cheers
