The difference in power between a .40 ball-bearing engine and a .46 ball-bearing engine like your two Magnums really isn't all that great. The two Magnums should weigh about the same, and they will perform about the same, even with a 15% difference in displacement between the two.

Any one of these trainers should fly nice and slow so long as the engines are fully broken in and they can idle at a nice low rpm level. You might like the way the Superstar flies a little better if you change the prop from a 10x6 to an 11x4 or 11x5 on the .40 LA. A larger, shallow-pitched prop will load down the engine and prevent higher rpms with the throttle wide open while not producing too much thrust throughout the power band. The APC 11x4 sport prop is a good choice, because it's fairly heavy for an 11" propeller compared to Top Flight or Master Airscrew props.

One of the reasons you're training with a flight instructor is because model airplanes are capable of fast speeds and keeping up with them is challenging while you're learning to fly. You can adjust your propellers and try to dial in your engine settings for good, smooth slow flight. You need to be patient though, and keep working with your instructor so that you can keep up with your plane.

What good would it do to slow your trainer down so much that you can only fly the slowest planes imaginable? If you have any interest in sport planes, aerobatics, pylon racing, pattern flying, combat, warbirds, or most anything else in this hobby, you're going to have to learn how to keep up with your plane. It seems fast now, but keep training and your instructor will have you flying your plane (as opposed to your plane flying you) sooner than you think.