cant get RPM readings
Yes, that would definitely cause some performance loss. On my Dust, I cut away the middle 1/3 of the firewall to allow the carb breathing room. I then built a little balsa air duct to help the air flow get down into the mouth of the carb better. The GRRT carb is really squished in there. Look around this site; there are a few good pics of D-Dusts with rear-induction engines and shows their installations.
Carl