If we are going to have a surplus of otherwised scrapped HSTs, 2+5 sets might do nicely for extra capacity on Sprinter trains.... although the lack of differentials might be an issue.
Journey times could still be fairly comparable if the 2+5's are better at getting to their top speed, even if they can't go as fast. (See the debate about Pacers Vs 230's).
One place that they could work could be on the SWR services to Exeter (although would probably need a 2+6). Yes you couldn't run then into/out of Waterloo in the peaks, but there could be some diagrams for which they could work.
For instance:
07:10 Waterloo - 10:40 Exeter
10:54 Exeter - 13:50 Waterloo
14:20 Waterloo - 17:42 Exeter
18:02 Exeter - 21:18 Waterloo
Just that one diagram would probably release a pair of 159's for use elsewhere (although possibly only only one as SWR would probably swap the pair from that diagram to replace two diagrams run by their 158's to then use that pair of 158's to run more 10 coach services which would release one 159 for use elsewhere).
In reality of guess that there's probably only 3 diagrams that could be released. The likelihood is that SWR wouldn't give up any more units to others than they could get away with (see above for where they'd get used). As such there may not be much extra for the rest of the network, but it would mean they wouldn't be looking to gain extra units from other parts of the network to provide a similar level of capacity enhancements.