Middle clicking (or shift+middle) to max upgrade your towers is very useful, specially when there is a ton of lag when premade party's rushing and speed poting consistently.
Lagging makes this much worse, imagine middle clicking while lagging and than trying to shift-rightclick to upgrade the tower but nope, the Gui will open 10 times reseting ur cursor.I'd just make it the same as with how you upgrade the mobs to a higher level i.e. have all the glass panes that you can afford colour orange.