To give priority to .tex files in Spotlight search?
It seems that you can't do that in Spotlight preferences. 

What you could do is reorder categories by drag and drop in the 'Search Results' tab inside Spotlight's preferences. You could also disable (uncheck) categories you don't want to like 'Applications','Suggestions'…

An alternative solution is :


from the Finder create a custom search that looks (criteria) for files with ending name : .tex 
Save this search, it will now be available form the sidebar of the Finder like a classic folder.
Select your saved search in the sidebar of the finder, then enter in the search bar the name of the file you want. The search will now occur only on the file inside the saved search. 


From the finder you could also do more quickly a search with two criteria :


Files with name ending by .tex
Files with name containing 'what you search'


Here's an article about custom search in finder.