The pilot's notes for the Mk 7 state:
"(ii) On selection below 430 knots deceleration is high (about 1G) and immediate.
(iii) Above 430 knots the airbrakes do not open fully and produce only moderate deceleration. As the speed falls to 430 knots however, they open to the full position, with a sudden increase in deceleration. If unexpected, this can be disconcerting"
As for maximum rate of descent:
"throttles closed, airbrakes out and maintain 0.8M/350 knots. These settings will give a rate of descent of approximately 20,000 feet per minute. Some misting up will occur."