This is actually possible to accomplish in the current system. When you type out "tex", instead of hitting Enter or anything, just click off of the dropdown.
It'll give you the stats for the [tex] tag and let you add it properly. This is similar to the same scenario with tag preferencesthe same scenario with tag preferences.
Even with the capability, though, I agree that exact matches should be provided in the dropdown. Here's a related proposal for this on all manner of tag dropdowns on the Q&A sites themselves: Please add exact tag matches to the autocomplete listsPlease add exact tag matches to the autocomplete lists. I don't know if the mechanisms are identical, but doing both at the same time would be swell.