Depending on chip it could well have say a 15 psi boost cut limit, some chips for RST used to have boost limit removed completely.

If it was blowing spark out it would misfire and splutter rather than backfire IMO.

When I had my RSTs in the 90s, I used Bayjoo chips from Harvey at SCS, used to hold 16psi with -31 actuator. Used to run NGK BCR8ES in mine. Gaps were 25 thou from memory, whatever that is in mm

Rev counter could be the cluster itself, or wiring fault from the coil.