Although that would be good considering how many issues the 810s have had, maybe the AT300s aren't suitable for level boarding with 915mm, or maybe the issue is with the platform height standard.
The vast majority of UK platforms attempt to conform approximately to the 3ft standard, mandated originally by 19th century regulations and fully permitted by EU TSIs during the UKs membership. While there are substandard examples, a ~3ft train floor will be a considerable improvement at those platforms than around 8 inches higher with conventional bodyshells, and raising those platforms to truly match ~1.2m train floors for LB will be far more difficult to achieve.
We do not need a new platform height standard. That'll just complicate LB for future regional trains, and end up forcing the 1.2m solution on every one of the thousands of rural halts around the nation. That's not practical or affordable I suggest. The bus industry managed universal low floor, and mainland continental railways continue to succeed in providing level boarding solutions at a range of heights, most rather lower than the UK standard. We have it easier than Germany for example which has a mixture of 550mm and 760mm platforms across the country both lower than our standard platforms. The UK must demand manufacturers adapt, just as they have on the continent, so we don't get locked into another 3 decades of no level boarding.
Level boarding provides advantages to operators well beyond the accessibility concerns and any associated legal requirement. With gap filler steps, the technique can speed up boarding, reduce the likelyhood of delay-inducing minor trip and fall incidents, and avoid difficulties lifting heavy luggage. Metro operators have understood this for decades. It could benefit a complex busy network like Cross Country significantly.
You could have the 80Xs split at Newton Abbot with 5 cars running to Paignton and 5 cars to Plymouth, with some running onto Penzance, which would also solve the issue with Saltash and only being able to take 2 or 3 cars when the signal is at danger. I would also consider if Paignton should gain another crossover so trains can depart Paignton on the Down and then cross to the Up to head onto Torquay.
I like the splitting idea as a method to gain more long-distance Paignton trains (I'm utterly biased of course, living in the bay!).
If the double to single points at the Goodrington end of the station were removed and platform #2 (aqua below) converted to a terminal bay, I think 260m+ standage would be available, with the buffers just clear of the signalling relay room. The track and platform could be straightened and extended to match the full train length so all doors could open. The remaining through platform #1 (yellow) could also be lengthened and straightened with its down signal moved closer to Sands Rd level crossing to also handle a full 10x26m train between signals.
In that scenario, the extra left-hand crossover at the north end is essential for full access to and from Goodrington sidings and to allow other trains to reverse directly in #1, while #2 is in use by a long express. LUMO has aspirations for running 5-car units to Paignton. A platform extension project, as described, could also allow them to run 10 car sets if desired on some services.
The aerial photo also shows why extending the double track further towards Goodrington to gain standage is not possible, becasue the alignment narrows at Sands Rd and the other track space over the crossing is taken by the Dartmouth Steam Railway single line. The signalling relay room is also a constraint, and there's a former crossing keeper's house the other side of the road, very close to the track that would also need removing to create room for an additional track. That sold for nearly half a million a few years ago (5 minutes walk to the beach!).
Track and signalling sketch:
