You can do a little work to the exhaust housing itself first to see if it helps.
Looking into the housing from where it attaches to the manifold you can see the sqaured opening the air must enter to flow through the WG.
Round out the edge nearet to the opening (closest to you).
This cuts down the sharp angle the air has to change direction before hitting the WG and may result in better control.
If you are talking about opening up the WG and installing a larger "puck" then richard Chan used to do it, but I've not seen much feedback about him lately.
Try contacting Reed at
www.turborepair.com
He does lots of work for list/bb members lately.