Lessee, the 100 hp Mark 5 was going to take the car to 200 mph and the 330 hp Mark 6 was going to increase that to 400 mph*, so 3.3x the hp gives 2x the speed.
The Mark 10 is 1500/330 = 4.5x the power of the Mark 6. It will go 4.5/3.3 x 2 = 2.75x the speed of the Mark 6.
If you remember the only reason they didn't run at Bonneville was they couldn't find a safety helmet to fit Harry, never said the engine was not ready.