The 5 Speed is a huge improvement!
I have an E9 with a 4 Speed and on the highway it is almost as obnoxious as my 1971 280SL!
On the highway at just normal speeds (70-80mph) both cars are screaming at almost 4,000RPM... The 280SL has very little sound proofing and it sounds like it is going to explode, unfortunately my E9 sounds almost as bad...
I often find myself checking to make sure that I am not stuck in 3rd gear. My 3.0CSi has a 5 speed and the difference is absolutely amazing...
The CSi is a MUCH nicer car to drive and not just because of the extra power...