I have a meth kit and mine stumbles a bit under partial throttle. I think we just need to balance the fuel a bit once the UTEC transitions. Sometimes I will even get a couple hiccups along the way but not always. It has a mind of its own sometimes.

 

My problem was bad plugs and they were gapped to wide. I went with 1 step colder NGK (BR8EIX). To make the transitions as seemless as possible also took allot of logging. Sometimes the timing and fuel difference change made the car stumble. You have to do a bunch logging to smooth it out.

The newest firmware release of UTEC for the subaru's has a Transition Smoothing fix. Hopefully they bring that over to the EVO UTEC.