It's a volumetric efficiency (VE) issue. They flow so much air at higher rpms that the turbo is unable to keep pace with that flow and therefore tapers. It's not "bad" boost taper. It's just a limitation of the turbo. Even so, IXs make way more power mod-for-mod than VIIIs with less taper. Don't worry so much about taper - worry about power delivery. Tapering to 14psi is more extreme than I've ever seen, though, so you may have a boost leak somewhere. Have you done a boost leak test?
The Forge WGA won't "relieve" the problem, because there is no problem with the stock WGA. There is typically more taper on IXs with the stock turbo due to how much that MIVEC head flows, but you should definitely check the system for leaks, since an 8psi drop is as bad as I get it at 6000' altitude. Hmm, isn't Reno around 5000'? If so, then that's exactly the reason right there. The high altitude means lower atmospheric pressure and less oxygen in the air, which means the turbo is even less efficient. My boost tapers from 26 to 16psi, but it used to taper from 26 to 22psi at sea level.