What is the top speed of the twin seat Hawker Hunter?
The manufacturer
lists over half a dozen variants of Hunter two-seaters.
You probably mean the T.7.

For a fair comparison, its closest single seat variant is the F.6, 
maximum speed Mach 0.94
(715 mph) at sea level.

The T.7A, a T.7 with different instruments, has a maximum speed of
Mach 0.92 (690 mph) at 36,000 ft.

So the T.7 is a few per cent slower than the F.6.
But it's no slowpoke: in 1959 it even won
the flying leg of a complicated race against 200 other entries.

Your empty-weight test suggests that these numbers are trustworthy:  the weights posted there say that the T.7 is 762 pounds (5 per cent) heavier.