Are you sure you are adjusting the right screw? Do you have the manual, or online info? I have an Izzy also, and from what I remember, there are 3 different screws that affect the trigger adjustments.
I thought when the Izzy came out, that Walther sold the same gun as a Walther model with a higher quality finish and laminated grips. I know I bought NIL grips for the Walther that fit my Izzy. I think the Walther model was a GSP2000 or something similar. Maybe Walther can source the parts for you.