Surprised to see 3+2 seating so popular in this thread when it seems so disliked on this forum in general.
I'm not an especially large chap but the idea of sitting in a fully (or even mostly) occupied row of 3 makes me very uncomfortable and would much sooner stand! With the exception of the aisle seats where I can sit half off the seat.
Certainly on the busiest trains I rarely see people making full use of the seating; in the bays of 6 I rarely see more than 4, which makes the whole thing pointless. The narrow aisles also restrict passenger movement to some extent.
Otherwise I think the turbos are great trains for Portsmouth-Cardiff; good luggage space, the extra carriages are badly needed, and having doors at thirds really helps dwell times, particularly as only a couple of minutes are booked at Southampton usually, and most passnegers change there.
TL;DR Turbos are good trains for Pompey-Cardiff but the 3+2 seating is no good for the many longer journies made, and I'm not convinced it adds as much capacity as it nominally would seem. Bring on a refresh with 2+2!