Storm, if you want to throw the car up on a dyno with a generic JWT tuned ECU, all the power to you. It's your money and your car.
Having said that, I must agree with what others have said. It is not a good way to spend your hard earned money. If by some magic you get lucky and the generic tune on the JWT ECU makes more midrange than the Marsh-tuned ECU, you will prove nothing other than the fact that the JWT ECU made more mid-range than another ECU that was tuned for a DIFFERENT setup. It will be nothing more than a coincedence of sorts. The point being, is that so many variables will be different and so a proper comparison can not be made.
Please also don't forget that you have a much stronger clutch package in your car that will change the ability of the car to put down power to the ground. Was your clutch not slipping previously?
I'd also like to make the point that you don't really have a Marsh-tuned ECU anymore. The set-up that Jamie tuned for, no longer exists. If you really want to see what he can do, take the car back and allow him the opportunity to tune for the changes you have made to the car. There is no way a generic and safe tune by JWT will outshine a customized tune (for your setup) by a well-known Nissan tuner.
My advice would be to stop trying to push everything so much. Just allow things to happen naturally. Take some time to make sure the new head gasket change will hold. Then, when your are confident with the car, have it re-tuned for the changes. Better yet, if you are getting a Nismotronic ECU, have it tuned then. John or Jamie would both be able to set you up nice. They could also tailor the tune for more mid-range pleasure. Do you have adjustable cam gears?
P.S. I have learned from many mistakes made in my own life, not to make decisions when I am mad or with similar emotions in the forefront. Take some time and re-evaluate your circumstances before making rash decisions.
Best of luck.