I doubt you'll find many small tanks with tempered glass, it costs a lot more to go through that process, and the glass needs to be cut to size before that happens. If the tempered glass would break, it'll just sort of explode, much more of a problem when we're talking about something that holds water. I don't know for sure though, that's just a thought.
I don't really think 5 or 2.5 gallon tanks need the plastic for support, but even with my 2.5 gallon there is noticeable flexure with the glass when the top has been removed. FWIW, I haven't filled it since removing the top rim, never took off the bottom, right now and forever onward it'll be for emersed growth. If by any chance that 2.5 gallon would break open it'll be too much of a mess. One gallon on a floor is terrible enough.
Anyway, ADA style rimless tanks are made differently, stronger silicone and thicker glass to withstand the potential for bowing. I'm not sure about the ADA tanks, but there was a lot of complaining (I suppose you'd call it that) on Nano Reef about the ADA tanks being tempered and undrillable.
In any case, I just don't think it's worth the risk, especially when the results are sort of mediocre anyway. The smaller ADA tanks really aren't that expensive and I do think they'd be worth saving up for if that's the sort of look one wanted.