Kenny Acheson went 253.5 mph in the 1989 qualifying. The particular C9 he was driving had a special chip that enabled it to develop more than the 850 bhp usually available for qualifying. Apparently, the Nissan R89C just got under 400 kph while Sauber and Toyotas (the latter with 1040 bhp in qualifying) went slightly above the mark.
In 1990, this Nissan R90CK (with at least 1100 bhp) reportedly hit 238 mph.... between chicanes.
http://www.youtube.com/watch?v=6lMItc4cy8U