Just had my Kadet Senior out today! That was my trainer and I still have fun when I fly it, although it HAS gone though some changes, like a '91 OS, flattened out wing and stuff like that. Two sides of the coin here, there's nothing like having a new plane to keep you interested in flying. My second plane was a Thunder Tiger Lazy Tiger with a 40LA. Whether you have another plane to master or not, keep trying things with your trainer. By the way, are you a "Crank and yank" kinda flyer? You know, wide open and always trying to keep up with it? If not, and you've learned throttle control, you are definitely on your way to improving your flying. If pulling down on the left stick is foreign to you, try it. Find out where the stall speed of your trainer is and learn to use the throttle and elevator to keep it up in the air. In my opinion (yup, it's only MY opinion so it won't get you much) you haven't outgrown your trainer until you've learned the ENTIRE envelope of what it can do. Many folks think that because they can keep it in the air and land with the engine still running, they have "mastered" their trainer.

Try new things with your trainer, if something goes wrong, your trainer has a better chance of survival than a faster, more aerobatic plane. You can even get a better feel for the different characteristics of prop sizing and pitch by swapping out the blades on your trainer, especially since you've gotten used to how it flies. A larger engine (you said it was underpowered) could change the flight characteristics greatly. If you are able to build, try building a wing with a lot less dihedral and fly with that for a while. It can totally change the plane and you've still got the trainer wing to use if a friend wants to learn. Whether you do any mods or not, try things like picking a spot on the runway and try to land consistently at that spot. That can teach you precision, even with your trainer. By the way, I was hovering MY trainer today.