Ya sounds like it is an old one, only one that is available aftermarket i think. the other ones ebay only. I figure if the oem got mikuni to change it maybe there is good reason.
most on here have used the keihin pwk 39, supposed to make more power.
I doubt anyone has done dyno pulls with the oem tmx.
I have jet cabinet full of mikuni stuff so I went that way.

Nothing wrong with using the stock one bored out or not. mine ran great with it too.
with my port jobs, leaning out the 0-1/4 throttle area was must. mostly by changing the slide.