Its not the afr's that kill the engine ................... it doesn't matter if you have a "richer tune" if it detonate's it will do more damage with rich tune than a lean tune.
Maybe your solution is to fit a better dump pipe then check / retune or not fit the dump and get a safer tune ?
I am not suggesting your tuner did not tune it safe but if you don't want to blow another cheap shit rb25 then get it detuned.