While the 15LA makes pretty decent power for a .15, the .25LA is definitely a weakling among 25s. Still, it will give you much better performance than a 15! Take the muffler baffle out, use a 10x4MA prop, and your speed will be a bit faster than a good-running 15, but you will have tons more thrust for launch, climbs, and continuous turns.

The 25LA does not have the same mounting dims as the 15, the case is wider and the mounting hole spacing is different.

Though it's a weakling, the 25LA is a nice running engine. For a little more than the 25LA you could get an FX which has gobs more power and is about the smoothest, sweetest-running engine of any size, it's a real user-friendly engine. Another choice is the Magnum 28XL, which is much more powerful than the 25LA, but not as smooth and user-friendly as the FX. Magnum has raised their prices and now the 28XL is not much cheaper than the FX, for the money, I'd go with an FX over a 28XL, but if you find a good deal on one, the 28XL is a decent engine. It does vibrate a bit and the carb bits tend to get loose and fall off, so make sure they are kept snug.

I'd avoid the TT PRO 25 and the Magnum 25XL, although they are ball bearing engines, they are relatively gutless and not much stronger than a 25LA, while being heavier.