Yeah, Id say try the turbo-jett to get more from the ST90. Works well with that engine. Generally you can go up one prop size, and keep the same rpm you had (say jumping from a 14x6 to a 14x8 or 15x6).

The ST90 is a pretty good engine, but if the plane really needs 108, 120 or 160 class power, i think the answer in that case is clear.

For an example of the turbo-jett installation, take a look at the following link. This shows a Jett 100L and turbo install, but you will get the general idea - its about the same size/fit for your application
http://www.jettengineering.com/edge/edge.html