Interesting--and it fits right in with the original reason for the 4.5 engine appearing at all, which was US emissions standards that came in 1973-74. MB tried to meet the standards with the 3.5 engine but couldn't do it with decent output and then-available technology; they responded by adding a liter to the capacity, and IIRC some tweaking of transmission calibration and axle ratio. The 4.5 SL was no faster than the Euro 3.5, and I think was in fact slower (not taking time right now to look it up) even though the output was nominally similar.
AFAIK, the only time a 4.5 engine puts out more than a 3.5 is if they're both non-US versions, which didn't happen much.