I don't believe the A4 was short of steam at all when the maximum was reached, so Mallard might have even been able to beat Mallard's record if there had been a section of suitable track long enough to keep going, but the record-breaking run really was a one-off publicity stunt and no-one else (in UK at least) was close at the time so I don't think there would have been any desire to repeat the exercise with all its risks and uncertainties.
Getting back to the point, I'm not sure if the valve diameter alone is the key factor in the steam path. The size of the ports and the size and internal streamlining of the pipes, junctions etc would all have their parts to play.
In terms of steaming rate, the W1 with its 50 sq ft firebox grate should certainly have been able to easily outperform the A4 at 41.25 sq ft. though, both at 250psi, which contributed to the W1's appreciably higher tractive effort that made it so attractive for the heaviest trains.
The 50 sq ft grate was carried through into the later Peppercorn A1 design which was very free steaming and successful all-round with 10" valves, but never noted for particularly high speed running, although that was probably far more to do with the ongoing post war austerity climate not allowing the opportunity than anything else. Initially, the LNER had notions of applying an A4-style streamlined casing to the A1 design, but clearly that was dropped before production.
I think the most important thing to come out of Mallard's run was the demonstration of the double Kylchap exhaust, which significantly reduced the back-pressure from the blastpipe. Gresley was a great enthusiast for this continental technology, but he was not permitted to equip more locos before WW2 because the patent licensing costs were so prohibitive. Once the patent had expired during the war, Thompson and Peppercorn were free to use it widely on the largest locos.