If you do the gbox might as well get a better clutch & lighten flywheel while your there, same price for labour.

The ECU is only as good as the tuner. You will always make abit more power with the aftermarket ECU, as the factory ECU is designed to be conservative, for the slight differences in different engines.

How much you make will depend on the amount of mods and condition of your engine.

If i were you i would run the stock ECU and wait until you have more mods before changing ECUs. That way you only pay once for the tune, instead of retuning for each new mod you do.