OK, so months after trying to get to the bottom of why my 1920X wont OC using P-State...
I finally got this reply from AMD:
To address your enquiry, AMD does not officially support p-state overclocking on AMD Ryzen and AMD Ryzen Threadripper processors.
AMDs motherboard partners have been able to enable this feature through their own engineering efforts. Due to how 1st Gen AMD Ryzen Threadripper 1920X and 1900X models are configured, p-state overclocking is not possible.
While not supported, this issue should not be present on 2nd Generation Ryzen Threadripper products, due to the updated algorithm used for Precision Boost 2 and XFR2.