Unfortunately I think 5 coaches would be too much of a leap from the current 2 (or even the previous 3). However, if the DaFT» insist on dabbling in rolling stock changes then I'm sure they could find a route currently operated by 3 car Cl 170 or CL159 (there's a clue!) services which could do with more capacity, thus releasing some decent stock for the Cardiff-Pompey.
Hourly 5 coach Adelante's on the Waterloo -Exeter anyone?
There wouldn't be enough capactity beyond Salisbury?