Couple problems with raising fuel pressure.
1. The adaptive nature of the EEC will tend to dial out the gains from increased pressure. This is not a reliable way of compensating.
2. As pressure increases beyond a certian limit, your flow will begin to drop off. If you raise the baseline pressure, then add boost pressure, then add the additional from the ZEX kit (it uses higher pressure to get you the additional fuel for the nitrous), you will be getting pretty high which taxes both your pump and injectors. I wouldn't suggest trying this.
You could mod your maf according to the instructions on MN12 Performance, but that won't help you run bigger injectors.
If you want to get a Mustang MAF, that is fine, but you need to make sure that you get the right calibration. You need to take the Mustang calibration injector size x 8/6 to see what injectors it is compatable with on a 6 cyl.
For example: 24 x 8/6 = 32 This is pretty close to the stock injector on an SC.
Another example: 30 x 8/6 = 40 This is close to 38's which is what I have on my car. Mine is a Pro M calibrated Cobra30 and it runs perfect.
Unless I am wrong, nitrous on it's own doesn't draw more air than when the motor is off the bottle. The additional air comes from the nitrous itself.
And what Steve is talking about with using different sample tubes to get more airflow capability is only valid if you have the ability to tune the EEC, which you don't.
As for getting more fuel by raising pressure, that is exactly how the ZEX kit works every time you kit the button. It does it for you. Technically you don't need to do anything, just like ZEX says.
But getting back to the original question, you asked if you needed bigger injectors. I said you should get some. You don't have to, but you should. Now you are saying that you don't want to get new injectors. The whole discussion about MAFs was centered around getting new injectors. If you choose not to get new injectors, then don't worry about the MAF at all.