The problems with my suggestion include reliability. But:
1) The shared pool on my mind would be based at Glasgow, possibly Polmadie or Shields depots. Probably a couple at Edinburgh would be useful as well, and on certain services to Aberdeen they could also prove useful, as these can be very busy at times.
2) They would run long distance Scotrails but more likely TPE Glasgow-Manchesters, where the higher capacity would be of benifit to the passengers.
This also allows a casceade of rolling stock in FSR, releasing 170s to replace 158s, which in turn replace many 156s which allows more 156 units to be used to lengthen busy trains in Strathclyde.
I believe, though, that there is a plan to move 170s to Shotts and use the released 156s to strengthen formations, so this would really be more useful in upgrading class 158 services to more comfortable and longer turbostar trains.
And I don't see why the SW still want them: they have huge reliability problems. Why not dump them elsewhere?