The 12x6 is probably just right for your plane.

Your LT 40 can do a lot more than most people think. If you practice and get really familiar it is capable of simple snap rolls, rolling circles, stall turns(hammerhead turns), knife edge flight and just about anything you can think of. Obviously its design limits you to how well you can perform certain maneuvers but it will do a lot of fun stuff.

Im not sure what your throws are now but once you get really comfortable you can increase the aileron throw to give you whatever their maximum physical limit for throw may be. Go all the way in on the control horn and all the way out on the servo arm. Hang on becuase its going to get really lively when you do this.
On elevator I would say you could increase the throw to about 16 degrees on your high rate.
On the rudder, again just move the pushrod all the way in on the control horn and all the way out on the servo arm and take all it will give you. You can never have to much rudder throw on a knock around plane.

Of course all of this throw will make it a lot more sensitive unless you can add a little expo so you will have to learn to move the sticks in much smaller increments.

This is where dual rates start to come in handy. Setup your rate switches with normal throw for take off and landing and then flip the switch and go nuts!
Just be careful when you 1st flip the rate switches and get the feel for how the plane handles before you start wringing it out.