Significant. I'd have to go upstairs,
find the right box, get out the flight
manual (Fred had a guy send me while I was
doing my Ph.D.), and learn how to use it
again to answer.
But, yes, hot air at high altitude hurts
jet engine thrust, a LOT.
I believe that pilots refer to this as density altitude. It factors greatly into allowable takeoff weight, takeoff distance and ability to climb over obstacles in the takeoff path. Pilots are or should be very aware of the density altitude on very hot days especially at high altitude.
But, yes, hot air at high altitude hurts jet engine thrust, a LOT.