Yes there is a ton of gain to be had. Reason i say that is because you have dual pumps and plan on putting bigger nozzles on.
30% nozzles are a poor financial decision. The parasitic loss of driving the second pump is barely made up for with 30% nozzles. 60% is the smallest you should run so you actually see some power gains from the second pump.
A 76mm atmo charger with 60% injectors and your existing 59mm HP turbo will put you in the 820-840 range with correct tuning.
oh. in that case doing larger nozzles with one pump will actually net you less power than now as the pump cant even keep up with 15% nozzles nevermind 30%. And no, there are no real gains to increasing the atmo turbo size on a stock fuel truck. You are using up all the fuel you have now anyway.