Until such time as the Waterloo - Exeter line is electrified then it is likely that the 159's will stay put. As it is unlikely that SWT's would go for getting rid of of their 3 coach sets which can seat 196 people in exchange for sets which can seat 169 people (i.e. 81 seats less per 9 coach train).
This is true, but because the 185's have a higher top speed, greatly increased acceleration and their dwell times are a lot less too, I would imagine this might free up another path between Basingstoke and Waterloo which would result in a significant increase in seats per hour between these locations.