Yes a larger engine will give you more takeoff performance!

But is it going to make it a different airplane not really, it is still a trainer with or without dihedral. The wing has a high lift airfoil (flat bottom) which somewhat limits aerobatic capabilities. You can go through a lot of trouble trying to modify the airplane to improve its characteristics...... but IMO its wasted $$$$ and effort. The Avistar is a good airplane a nice trainer, but it is what it is! The .40 LA teaches you to better fly the wing vs. flying an overpowered thrust vectored airplane making you a better pilot in the end.

I would go ahead and purchase your choice of a more advanced airplane/engine and keep the Avistar as it is! Everyone should have a trainer in their hanger IMO, there is something nice about relaxing flying with a trainer, good for buddy cording friends, or just doing touch and go's when the mood suits you.

Welcome back!