Originally Posted by Gilly

MB doesn't give 1/4 mile time, but all 560SL's are listed at 7.7sec. 0 to 100kph (roughly 0 to 60 mph). 560SL was US, Japan, and Australia only.

Lot's of different info for the 560SEC as there were versions with or without catalyst.
From 10/85 to 8/87, ECE version 0 to 100kph 6.8 sec, non-ECE without catalyst 6.9 sec, and with catalyst 7.6 sec. No ECE versions are shown after 9/87, I assume it was dropped.
As of 9/87, without catalyst 0-100kph 6.8 seconds, with catalyst 7.2 seconds.
The rear end ratio was different between the SL and SEC (2.47 for the SL, 2.65 for the SEC).
It looks like the top speed of the SEC is also higher than the SL, although the SEC is a heavier car. Looking through the other stats, the 560SL is rated at 230hp, and it varies for the different set-ups on the 560SEC, but the SEC versions are all higher hp than 230, so that is the reason for the difference.
Gilly
PS also noticed different ratios in the transmission for 1st and 2nd gear, 3rd and 4th are the same though.
PPS From 9/87 the version WITH catalyst had 279 (!) hp compared with the 230 for the SL!!