I think the simplify 4 patch had some more improvements for contourlines - but then I played around all the time with the DP values so it's hard to compare. I do know that simplify v4 versus 3 back then was an improvement.
I think merging lines of different directions would be good too - with the caveat that we would need an additional instruction in the lines style-file to tell for lines not to merge (maybe make this a list - because then it's not needed each occurrence in the lines file, but once per type. I feel this is only important for any type that is either off center (e.g. I have MTB routes on the right side of the center line, hiking routes on the left - so you can see both if on the same way) or that have an arrow or similar in the typ-file (i.e. I have arrows on my rivers so you can see the direction of water flow). Oh and not sure if this is would be merged also at level 0 or only >0. That would make a big difference in how many line types I put on that list. I mean there are many types also at level 0 where I don't care about the direction. Of course any line with oneway that is routable could not be merged too. On the other hand even quite a few routable lines could be merged - but not all.
If this also applies to level 0, then that file needs to have one command for level 0, and one command for level 1 and higher. At level 1 and onwards it only depends on the typfile if merging is possible or not. At level 0 it depends on both typfile and oneway attribute.