Why would FGW even think of lengthening them? As I see it with Thames Trains they worked far more services - everything at the moment + Oxford fasts, Cotswold line, Oxford - Bristols, Paddington - Sloughs. Now they have none of those extra services - Oxford fasts are are 180s & HSTs, Cotswold line are 180s, Oxford - Bristols are long gone, Paddington Sloughs have morphed into HConnect with 360s. New services? Well they're going half hourly to Gatwick - but with PSRs or their equivalents (meaning they cut stations instead of running more services) mean that those services shouldn't need many extra units - I guess 1 at most. The Turbos are now running in multiple much more frequently - most welcome - but I doubt the extra capacity that they might want in the peak could be justifed at most times of day, and so there's not much incentive to build anything new. Not to mention that FGW are unlikely to be spending money on much except their franchise commitments..