First problem is proving that your two different propulsion systems were working equally well. Unless you know that for a fact, all the other questions are just speculations.

For example, "the wieght of the plane is actually less....etc" And did that move the CG? If you didn't move stuff to compensate for the lighter nose...... you created a tail heavy airplane. Or one that's more tail heavy. And that gives you a more effective elevator if you're lucky. And a over-sensitive airplane if you aren't. Which will fly erratically if you're lucky. And crash if you're not.

The geared original was swinging only a 6" prop? and the outrunner a 10"???? Usually a gearbox is used to swing a larger prop, and does it more slowly. Sounds like the difference in power was considerable. And you mention, "the outrunner motor and lipo battery." So you also are using a lipo too? How much lighter is the new combo than the old one?

If the CG moved, it moved back. The elevator became more efficient, and if the CG went too far, the plane went unstable.