In general terms, for performance,the less back pressure the better, as long as the intake and fuel can keep up with the exhaust. The performance loss that some people encounter when they upgrade their exhaust is almost always attributable to a lean condition due to the fuel system being unable to keep up with the increased airflow through the engine.
Also, electrons move faster than your engine can rev. If your tach "reads" slow it is not because the engine is too fast for it, it is because the manufacturer has built in a little electronic damping to keep the needle from jumping around like crazy. Independent of the tach itself, there is a rev limiter built into the engine management system that works in real time and will step in and cut fuel or ignition as needed to keep the engine from over revving regardless of the tach reading. The only way to over rev the engine is to mechanically over drive it by downshifting to a gear that takes the engine above red line, or running it up to the verge of red line under load and then suddenly releasing the load while keeping the throttle pegged, thus allowing the engine to "spike" in rpm before the system can shut it down.
This only applies to modern fuel injected/electronic ignition vehicles. Old school stuff with simple ignition systems, carbs, etc. will rev until something gives as long as the throttle is open enough to let it happen. Ever seen an engine blow at a VW meet?
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