3-car Class 159 has 172 Standard, 24 First. 3-car Class 185 has 154 Standard, 15 first. Can't find the seating capacity of a SWT 2-car 158
There are 51 185s. SWT has 30 159 and 11 158
So, if SWT took the 185s, it would be a larger fleet. However, if used as 3, 6 and 9 car trains as direct replacement, they would have lower capacity. Worse, 10-car formations would not be possible, and 12-car would be too long for the platforms. The Guard would only be able to check and sell tickets in one unit and if catering trollies were operating, there would need to be up to three per train- unless there was a hilariously expensive rebuild to change the cabs to a version of the 444 or 380 cab (not impossible, just very, very expensive).
It would mean more capacity on the local services currently operated by single 158s, and would potentially allow more 3 or 6 car services to be extended.
Overall it's probably not a good solution. It's basic attractiveness is "SWT use Desiros. 185s are Desiros, therefore they should be at SWT"