Actually, you can research up to level 9 in T/R weapons. Levels 7, 8, and 9 give you the "Small Graviton Beam" levels 1, 2 and 3 respectively. Assuming you have researched Smaller Weapons 1, 2 and 3.
BTW - the damage stats on the Small Graviton Beams are pretty good - I'll have to research those. Thanks for the tip
