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.
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
swappage out its least-recently-used pages to swap.
Long answer is the same. ;)
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.
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 your system and use that.
Without knowing the details of your performance needs, it's very hard to answer your question with specifics.