Let me repeat the points I made some time back.
There are not enough GWR 158 to go 5 car on the Pompeys. If more were available, that would have been the first option. But there aren't and no-one was willing to give any cl.158 to GWR either. So a different traction choice had to be made.
The DfT were (and are) totally fixated on seat numbers at Bristol. So that is why the 2+2 conversion hasn't happened, yet. Too many Bristol peak seats lost for their liking. The route serves two major markets, the Bristol "commuter" flows as well as the more longer distance Pompey-Cardiff type punter. The DfT made their choice, effectively favouring the needs of one rather than the other. It is something that will be revisited in the DA3 discussions.
The Turbo refurbishment saga is a very sore point with GWR. The DfT didn't think it offered value for money in DA2 and only now, with the units in a not too great a state - to put it mildly, are they willing to consider a proper refurbishment plan for DA3. To those who have seen the Turbos since new, the last few years of rapidly ageing interiors have been rather painful to watch.
The route is not an easy one to time. Short HST sets aren't really practical for the point to points, even if you could get them cleared (and SSL bogies are the least of your problems in that) and 150/158 combos restrict you to 75mph where you really want something that does 90mph, especially after the revised IET timetable comes in.