I can safely say Cambridge does not need an intercity service. It would be overkill, off peak all is needed is maybe a couple of 8 car trains on the non-stop services. Even that wouldnt be vital at all, but would be welcome. Capacity is definatly needed in the peaks, but not new trains. Just longer 365s, the speed is fine as north of Hitchin line speed drops and the door layout makes them ideal for when they run semi fasts. I know there aren't any more 365's but we certainly don't need HST type trains, mostly because all trains terminate at Cambridge. It's only the Kings Lynn and the XC services that regularly run as through services off peak, and only a few GA to Kings Lynn.
You're absolutely right. Oxford is different, since the Oxford Flyers can run through from the Cotswold Line and have to mix it with HSTs for the 40-odd miles from Didcot to Paddington. Something 395-ish might work, but only if it has inter-operability with other stock on the route. I'd prefer to see the Cambridge fasts running into Liverpool Street, so that they don't take up more paths through Welwyn. Still, that's probably impractical, and the current solution just happens to be what works best.