Not enough to really worry about. I'd be more concerned with reliability (loom/ECU, CHG etc) as this would pale any fuel consumption differences into insignificance..!
2003 W209 CLK, 2005 R171 SLK, Volvo XC40 P8 Twin Motor Pro EV
Recently checked my S124 E280 on a run back from Italy. About 28mpg overall with a best section at 32mpg. I think its marginally better than the 300ce but not significant in the real world.
Thanks, I know the loom has been done and there are 110k on clock with FSH- it really should be trouble free but maybe I am spoilt with the coupe- haven't had any big expense in 10 years- just too much rust and not a lot of space!