Theuns,
I just measured the inlets and tailpipe nozzle on my BVM Maverick Pro and the inlet radius is 5 mm and the nozzle diameter is 95 mm. The inlets have a splitter plate right up to the fan face. This Maverick has been flown extensively with a BVM 81, K&B 82 and a Fan-Jett 95 engine. It has been radar clocked at 165+ MPH in a racetrack pattern-no diving. I don't recall the takeoff RPM but it was just below peak where a quick pinch of the fuel line would yield a rise to the peak. I did have a special tach with a LED and sensor to read the RPM from inside the fuselage. My fuel mixture was similar to yours except the oil content was increased to 23%.
I recommend trying the 95 mm tailpipe and fabricating a splitter plate inlet. Be sure to secure this plate well and apply several loops of CF tow on the inlet exterior to prevent collapse due to suction.
Note that I have about 20 years of ICDF experience on various airframes including the BVM A-4 Skyhawk, Sport Shark/ Aggressor and Maverick Pro plus the JMP Starfire and DL Cyclone. Good luck with your Mirage 3 and please post your findings.
Regards,
Art ARRO