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.
Short answer: no, there's no Tmpfs for Mac OS X. Tinkering with RAM-disks in Mac OS X is just naively mimicking what you get with Linux Tmpfs if you consider Linux' version as archetype — simply due to Tmpfs doesn't cut specified amount of RAM from system memory, allowing to
Long answer is the same. ;)