Might want to think about limiting boost in the lower gears, by using two different boost controllers with different settings, (or other method) anda valve that switches between them as you shift up in gears. One low boost setting until you get up into 3rd gear and then the other in the longer gears when the engine is under full load and the turbo is fully spooled up and you can actually use the extra power.
Maybe a cable controlled mechanical limiter that holds the waste gate partially open in low and second gear.
Some of the more advanced electronic controllers do that to limit wheel spin in drag racing, but you can also do the same thing using only mechanical methods such as valves on the waste gate controller or different waste gate springs or bypass plumbing etc.