As for how much can be extracted is limited by your budget and the tuner's talent. j/k MB was more forthcoming with power specs on W140's than the W124's for some reason. You do get more power out of the box with the earlier cars. If you look at Renntech's W124 page http://www.renntechmercedes.com/w124.html
You will notice that the early 500E's can be modified for power much easier than the later cars. Both cars can be taken to the same level it just costs many thousands more to do to the later cars. ex. a stage 1 6.0 engine is $19K on the early cars, $27.5K for the later cars. Since such grand upgrades are likely out of the question for most of us mortals it would more specifically fall to chip tuners and the like. Since everyone who tunes MB software is different (and secretive!) it is hard to say. It would stand to reason that the earlier cars could be tuned to a higher output number. Keep in mind, they are faster to begin with.
I do suspect that the difference is bigger than MB says it is. Ex. MB claims 93 500E's had 315HP and 347lb-ft while 92's had 322HP/354lb-ft. Having driven both 92 and 93 cars the difference is bigger than 7HP and 7lb-ft. On 3700-3800 lbs that amount should be negligible. If I had to guess I would say that the 93's did make 315/347 but the 92's made quite a bit more than 322/354. I'm talking about a difference you can easily feel and measure. I was feeling/noticing the difference long before I knew of the "feature".
I have noticed that the later cars are more stable in their performance. No matter the outside conditions, they perform about the same. The earlier "hot" versions seem "turbo" temperature sensitive. I have measured up to 1 full second difference 0-60 on the 400E. That was between a 40 degree F evening and a 100 degree F afternoon.
Hmmm...if those early cars are that sensitive there has to be a way to trick the computer into thinking its colder outside than it really is! That is about all I can say on this matter...