Let the numbers speak for themselves:

Kleiner Kurs Hockenheim
SL55 AMG - 1 min. 18,4 sec.
Boxster S - 1 min. 18,3 sec.
BMW M3 E46 - 1 min. 17,6 sec.
Honda S2000 - 1 min. 18,9 sec.

The Boxster S beats the SL55 AMG.

A even better example:

SL55 AMG tuned by MKB (558 HP) - 1 min. 16,5 sec.
New 911 Turbo - 1 min. 14,6 sec.
Ferrari 360 Modena - 1 min. 15,1 sec.

Even tuned, no sportscar.