From experience, I use it quite often and there are a few things you can do depending on time of year and how far in advance you book. As with the majority of GWRs advance fares from Cornwall/Plymouth to and from Paddington, they seem to save very very little in recent months and for return journeys then a walk up return is now normally cheaper than two restricted advance singles. If you do find an advance fare for the sleeper that does offer a significant saving against a super off peak fare then you can just buy the advance fare and then call GWR or go to a station with your ticket and buy a berth separately, though it will be at the fixed price advertised (usually £90 minimum for solo) as if you were upgrading onboard on standby. Both of these options are subject to availability of course. The last few times i have used it i have found it cheaper buying a super off peak fare on GWRs site or browser (NOT APP) and the berth upgrade in same transaction, this way can be as low as £40 for a solo (that is what i have paid for November 5th) A super off peak return Plymouth to Folkestone and a berth upgrade for the return journey in evening. At the time i booked it a month ago, two advance singles saved less that £2 against a super off peak return, plus the berth would have been another £90 minimum.
Edit, just notice you were booking a twin so ignore the sums i posted as they are solo prices, but the theory is still the same that berths booked well in advance as part of a super?off peak ticket are usually substantially less than the onboard or booked separately price. Are you only making a single journey?