It's not true that the HH5A doesn't support g.inp with one proviso, it's downstream only. I have one, and it does, and have tried both an A & B (don't ask why I have both - to do with moving and currently having two properties). Both give similar downstream speeds and rock-solid connections (about 56mbps on an 80mbps service). The B does give about 1mbps more upstream though.
When g.inp was enabled on my local (Huwai) cabinet, the change was very noticeable as prior to that I'd average one disconnect a day (now its only on the fortnightly scheduled modem reboot - and the B does the same thing). However, the first sign had been an increase in sync speed followed, within a day, with a reconnect with interleaving turned on (which lost a lot of speed) and then, about a week later a resync with interleaving turned off and the return of the lost speed.
The g.inp situation is a lot more complex that HH5A doesn't support it.