Hi Dan, I called around a while back and spoke with various vendors about there turbos. Turbonetics was a little difficult to deal with in obtaining information until I managed o get past the sales drones and speak with a tech type.
From what I gathered, the stage 3 "Cheetah" is roughly equivalent to a TA-49. They don't recommend an upgraded converter but do recommend larger injectors. They no longer list a Stage 1 or 2. The Stage 4 is similar to a TE44 and the stage 5 is similar to the TA 60/60-1.
For in depth Turbonetics info I would highly recommend you contact Innovative Turbo and speak with Rick Head. He used to work for Turbonetics before spinning off and starting his own company. He's a knowledgeable guy and friendly.
Good luck,
Charles Brooks