Here's what I think:
1) Same allowances as non-turbos. If stock ECUs cannot be policied on normally aspirated cars, then they can't be policied on turbos.
2) You can't legislate max boost, as it is an effect of some things, not a modified part. It cannot be measured.
3) Use a TIR to make the horsepower output predictable. Making a big assumption there, I have a lot to learn.
4) Assign weights based on that predictable horsepower and our class' power-to-weight ratio targets, same as any other car.
5) Turbo diesels would be allowed too. They would probably get a torque adder like any other high torque motor. Diesel non-turbos could be classed right now, except that I'm not aware of any that are worth classing.