RE: props for Jett .35
sorry to hear you lost the first plane ..... but you know what to expect now !
I am confused ... briefly, did you order the pipe tuned by Dub along with the engine, or are you going to add the pipe later?
The FIRE rear exhaust engines typically use a full pipe. Dub's pipes are slightly modified to help with exhaust velocity and backpressure.
Dub typically pretunes his pipe systems per your prop/application request . Once Dub has the system tuned .... he should let you know what the rpm target is. From there, you can play with props a bit, with the goal of loading for the same ground peak rpm.
If you build the plane without landing gear, you will probably want to stick with the stock 9x6 for a while. You need the immediate thrust for hand launch.
The racing props are a good alternative for better top end speed, but the plane will not accelerate out of your hand quite as rapidly. Smaller 8" diameter will have the same issue.
As far as aircraft performance ..... you are doing about all you can there. It has a fairly thick wing, and a good deal of frontal area.
Compairing performance between the red muffler and a full pipe - gain is about 500-700 rpm more on the top end. Change in aircraft speed for your Kobra would not be substantial.