With all that power underfloor. I can't help thinking the S and C or scottish hills
The S&C would be good terrain for them, but that'd only ever need a handful of DMUs - it's a bi-hourly service and the journey time is only around three hours, so it's never going to require many 185s.
Three coach DMUs will be plenty of capacity (beyond maybe a doubled up unit leaving Leeds on weekend mornings and Carlisle on weekend afternoons during the summer), so I don't think you could even beef up the numbers by doubling up most diagrams.
I'd suggest 185s on the S&C only if a significant number were going to be used on other Northern services around that area (whether Carlisle - Newcastle or Leeds - Calder Valley - Blackpool/ Manchester). Otherwise it'd be a micro fleet.
On the subject, interesting to note that TPE suggest that using their services (and changing at Manchester) is more convenient for those doing the Leeds - Carlisle journey:
Jumping on board a Transpennine Express train is the most convenient way to travel from Leeds to Carlisle
https://www.tpexpress.co.uk/routes/trains-from-leeds-to-carlisle/
Shoehorning the 185s into the SWT franchise seems a little bit convoluted from what I've read
Yes, and it does seem to be a bit of a Forum obsession (see also "Shoehorning 222s into the SWT franchise" etc).
158/159s are perfect for the SWT franchise for the time being though - given the need to fill platform space at Waterloo, the need for portion working, the need for flexibility. Keep them there until they are life expired and can be replaced by a new generation of 100mph bi-modes.
Keep the 185s somewhere that doesn't involve splitting them into more than two fleets.