Is G-23 and G-38 mount bolt pattern the same?
An 18x6 prop on a G38 is seriously underpropping it. You are stressing the rpm capability of the engine and are well over the torque curve peak. Put an 18 x 10 on both and you will see a difference the other way. That is seriously overpropping the G26. Is apples and oranges.
However, the comment that you might want to consider the G26 is very valid. People say good things about them.
Gerry Rosebery