I think it would be expensive to do that. They would then need to have 2 engine programs. Much cheaper just to pop a plate and gasket in the carb.
I know one of the biggest complaints the drivers have with the plates is not just top end speed, but it also reduces "0 to 60" speed. The cars become sluggust and not as responsive. This requires them to stick together, so they don't loose the draft, even during pit stops.
I remember a few years ago they tried to open up the plate to give them more HP, but added a rail on the roof of the car to keep the top speed down. I thought it was a good idea and the racing was pretty good, but NASCAR abanadonded it.
