If you keep your glass tops clean, a 10-20% loss in lighting intensity seems well worth the advantages of having them (fish staying in the tank, less evaporation and better insulation).
Certainly, there are situations where the downsides outweigh the positives of glass tops, but you're rather unlikely to be struggling with an overheating system with LEDs.
Do keep in mind that LEDs are very sensitive to overheating - I'm not familiar with the Mazarra units, but placing the fixture directly on the glass top is probably unwise.
If you don't want to hang the fixture, I would elevate it off the top with legs so you can get some more airflow across the emitters.