Your 300 feels weak because like all Mercs of that age, including my 190e, they pull away in second gear rather than first as standard.
There are a couple of ways of getting round this. With the S/E switch in the S position, when you want to pull away quickly, push the throttle down hard to the stop, it will then kickdown into 1st to pull off.
Or when stationary in D, pull the shifter back to 2 then forward into D again. This will default the box to pull away in 1st nomatter what throttle is given. You are gonna have to do this everytime you want to pull away in 1st though.
What you will find though is first is very very low geared, and it will quickly shift into second with a light throttle. Floor it in 1st though, and you will be pleasently surprised!