Hatfield tests on HJ679, no tanks, ejector exhausts, +18lbs boost, all up weight 19,100lbs:
5,000ft: 371mph
12,400ft: 384mph
Karnak's tests on Aces High Mosquito Mk VI, no tanks, saxaphone ducts, +18lbs boost (gauge read +14lbs at 50ft and 5,000ft and +10lbs at 12,400ft, but I think this is an error in the gauge model, like it is using a Merlin 21 or 23 FTH rather than the Merlin 25s its performance indicates it has), all up weight 19,100lbs:
50ft: 338mph
5,000ft: 362mph
12,400ft: 376mph
If those ratios hold, HJ679 would do 347 or 348mph at 50ft. Still a bit slower than the claims of the Mossie 6 pilots I have had the luck to recieve replies from. Each of them, with one exception, said it would do a little over 350mph at sea level. The one exception claimed over 370mph, but I think he was looking at one of the air speed indicators that de Haviland had warned would read about 20mph too fast at high speeds.