There is always the possibility of re-forming the Turbos to produce some four-car formations for Cardiff-Portsmouth by shuffling around centre cars. Time will tell, but I can't see other operators rushing to surrender any 158s to FGW with Pacers to replace, so Turbos may be the only way to get extra capacity on that route any time soon.
It been doing the rounds that the current plan is to split the 158s back into 2 car units, and use them doubled up on Portsmouth - Cardiff to make them into 4 car trains, with 16x's helping out
16x's will have their coupling wiring and software altered, so that they can work with 15x units (but not pacers).
150s are unfortunately, likely to stay on Plymouth - Penzance workings (and branches) to begin with when Turbo's move to Bristol, and some 165s will work in the Exeter area.
Post Cornwall re-signalling, there is a possibility of an hourly Bristol - Penzance local service, as Extensions to the current Taunton Service. It may look like this post 2020 (Padd services not included)
1tph - Cardiff - Portsmouth to stay as is and run as 2 x 158s with the occasional 166 to cover for unavailable 158s.
1tph - Swansea - Bath EMU via BRI. This could potentially become a through E-W service in the future. This replaces the Cardiff - Bristol Temple Meads section of the Cardiff - Taunton Service
1tph - Bristol Temple Meads - Penzance stopper (most shacks to Penzance)
1tph - Padd - WSM via BPW
1tph - XC NW - SW
Selected XC MAN - PGN/EXD/PLY/PZN
1tph - BPW - WSM (all shacks)
That's just between Taunton and Bristol. I would imagine if the proposal for an hourly Bristol - Penzance service went ahead, 90mph stock would be used.