Class 380s have operated on the Cathcart routes on a daily basis for over a year now so I doubt there's an issue with overheating. Class 334 units were used as stand in units back in the day and again if there was any issue, I'd have thought there would be an operating restriction placed on them. Much of the North Clyde routes have a similar profile in terms of distance between stations so the units' traction motors will be getting worked hard there also.
The issue with class 380 (and class 385) units on the routes are mainly internal. No need for tables and the doors are too far apart. Aside from that they are fine in terms of acceleration. Class 318 and class 320 units are too cramped inside. Class 334s, especially if increasing the number of 6 car workings, would seem ideal. EDIT: I see the reference to the traction motors on the link above - maybe there is an issue with prolonged usage, but there has never been an operating restriction placed on them that prevented all day working on the Cathcart routes.
One issue with introducing the 334s onto the East Kilbride and Barrhead routes are whether conductors could work them. They are very much driver only units, which will not likely be introduced onto those lines without likely industrial action by the guards. Introducing driver only operation out of Glasgow Central any more than it already is would effectively see the end of the grade at Glasgow Central - there's no way it would be accepted without a considerable battle.