The "rumor" about Silvermine comes straight from the Z Power Steering site, but it isn't necessarily wrong. I believe Silvermine uses an EZ-Steer kit to make their units. Those are designed for ATVs that weigh more than most Zs and experience a lot rougher roads than a car does. They likely don't have the factor of safety of an OEM automotive unit like the ZPS, but it's up to you if that matters. I have seen multiple people who run the Silvermine kit on track and have never seen a complaint, and the EZ-steer kits get used on street rods a lot. If there were an issue with the SM units, I would have expected to see some firsthand accounts of failure online, but I've never even heard secondhand of one having issues.
Here's a discussion we had years ago about delay with these units.
Interestingly, the only complaints were about force-sensing units.
The GPS is also only necessary if your car is stock without an electronic speed signal. I'm not sure how your car is setup, but with a VSS signal, there would be no chance for lag. If you have an aftermarket ECU, you could even setup a custom assist curve if you wanted.