temperd glass can be cut but it is very very very difficult. if the glass get hot in any way shape or form it will shatter. a very good sand blaster could do it but they wont because it will take all day to cut it with sand and it might shatter and they dont want the mess. my opinion is get an overflow for 80 bucks and dont attempt to cut the glass. oh btw, i have an "all glass aquaium" and it says right on the side with a sticker tempered glass do not cut. some manufactures i guess only use it on the sides and others use it all aroung the tank.
but that my$.02