I run a maxed out Rex quite a bit (usually with a full clone team)...I think he's extremely solid where he's at. Consider that the primary synergy trait of clones is, in fact, TM control and assist attacks. When you see how he fits in with the other clones he is very effective.
Don't underestimate how Impeding Shot, Squad discipline, and Brothers in Arms helps change the tempo of a fight. When you see how many extra basic attacks he can sometimes pull off when the full clone synergy kicks in or how often he generates clutch attacks and wards off special damage you may reconsider.
You do bring up a valid point reference the Subdue attack. It can be handy when you need to burn down RG or other tanks but most times his basic attack is actually more valuable. I would actually be ok if he dropped the attack and added a party heal on par with JC. I would use basically the same skill ' animation as the Stormtrooper Commander's green healing "grenade". Echo can do wonders to Clone survivability but they lack the burst heal that can pull you through a power hitter on the other side.