Try mounting the .28 inverted on a test stand and see how it runs. You might find that it's troublesome even on the stand. If it's sputtering, see how it responds to adjusting the carb. I'm also still curious what you are using for a plug.

The difference between .36 and .45 is actually quite a bit. I've got several of both size. The .36 engines are upsized .25s. They are quite a bit smaller and usually several ounces lighter than the typical .45 engine. For example, the TT .36 Pro, a very popular .36 engine, weighs in at 11.4 ounces, while the OS .46 AX and TT .45 Pro are both over 17 ounces. Adding 6 extra ounces to a plane this size is a significant difference, especially when you consider the extra lead it would need in the tail for balance, and the very thin airfoil. The overall effect on the flying characteristics is not going to be positive on anything other than the vertical.

It's also hard to imagine that you will even be able to use the extra power without tearing the thing apart. Mine's a rocket with a Webra .32, and I've got it pitched down to reduce the speed. Much more power and the plane would be subjected to stresses that it probably can't handle.

Personally, this is one I wouldn't overpower. But, it's your plane and if you're happy with the .46, great!