Due to the performance mods that I've made my back-pressure at low RPMs is kind of high, so the valve starts modulating sooner than intended.
Yeah, because you measured it with a backpressure gauge and compared the results of the original exhaust system to your modified system.
What I am wondering is if I could take a valve from a 4.5 or 4.9 and get higher mechanical resistance to the back-pressure, so that the valve did not come on until later.
Your answer: No. The EGR opens when the ECM tells it to regardless of how much backpressure is present. As long as there is enough to allow the vacuum from the solenoid to lift the diaphragm, the egr operates as it should. Only too little backpressure will interfere with normal EGR operation.