How much thrust is lost through a curved exhaust nozzle?
For the F135 engine in this photo, thrust in hover is only about one per cent less than maximum thrust, if Pratt &amp; Whitney's data sheet is to be believed.

Maximum Thrust Class 41,000 lbs
...
Hover Thrust 40,650 lbs

The Hawker Harrier's maximum thrust was about 20,280 pounds.
An approximation of its maximum vertical thrust at low airspeed is given by its maximum vertical landing weight, 19,918 pounds, again just about one per cent less.