Glad to welcome another member to the Evo family
You can fit a Power Commander but, unless you disable the lambdas, it will have no effect during closed loop running as the lambdas will constantly be trying to bring the Power Commander's AFR back to 14.7/1. Also, if you're going into the ECU to disable the lambdas, you might as well get the reflash too, (which also disables both the lambdas and exhaust butterfly valve).
The Rexxer reflash will also be cheaper than the Power Commander, especially if you factor in set-up time.
Let us know what you think once you've got it - and don't forget the photos!