I think my HH5a must have had G.INP enabled downstream even though the formula says it's not.
On Sunday before my HH5a re-synced the maximum sync rate it ever managed was 60Mb or 55Mb on my HG612. Then after the re-sync the HH5a synced at 65Mb. For interest I swapped the HH5a out and installed the HG612... it now syncs at 61Mb and reports that G.INP is enabled.
For me that's enough to suggest to me that my HH5a is capable of G.INP on downstream.
Plusnet unlimited FTTC load balanced with BT Infinity 2 and BT TV