The hp/lb of boost ratio tapers off as the boost is increased, in this case, because the turbos begin to make more heat (enter a lower area of efficiency)... so right now they are making about 13.3 hp/lb of boost... say it continues that ratio until 25 psi, netting about 640 rwhp. But after that it will begin tapering off so he might be making only 8-10 hp per lb of boost... and continue at a varied rate so that raising the boost won't increase power because the heat offsets the difference in cylinder pressure.