That is: a file system designed specifically for ramdisks? I know I could use any old file system but I want something specialized for performance.
Once you have a ram disk device, you can put any fs on it you like. HFS+ is going to be at least within an order of magnitude as good as any other fs. Since you're on a Mac, it's the most natural FS to put on your ram disk.
That said, ZFS is a very efficient and highly optimal FS, as well.
But as stated in the other (duplicate) question's answer: unix is pretty darn good at caching files with memory it's not using for other things. Taking space away from your OS and dedicating it to a ramdisk is usually not a good plan.
If you want to lower your i/o wait times and you have a very specific need, perhaps add an SSD to you system and use that.
Without knowing the details of your performance needs, it's very hard to answer your question with specifics.