For some further reading on the effect of wind on instantaneous airspeed in the case of a turning aircraft (the aircraft having inertial mass - 'inertial' for emphasis, mass would suffice of course) have a look at Dynamic Soaring.
This is a technique of imparting energy to an aircraft (or bird) by utilising the differing wind speed in different envionments and by manoeuvering the aircraft between the environments. Apparently this is the method that albatrosses use to cover 1000s of miles across the oceans without significant effort. In the latter case the variation of wind speed in the troughs and crests of waves is utilised.
The speed record for radio control model gliders using this technique near mountain ridges is apparently 392mph, so the effect can be seen to be not small. This is *sustained* flight - as long as the wind blows. Yes really!!!!! From an on-line video of 370mph I estimated that the model was pulling 35g - I am not 100% on the integrity of the video or my calculations.
There seems some interest in Dynamic Soaring within the full sized glider community.
Dynamic Soaring links
YouTube - Great sounds of formation DSing at Jones Pass
YouTube - New Dynamic Soaring World Record -- 392mph
Dynamic soaring - Wikipedia, the free encyclopedia
ESA Experimental Soaring Assn
http://www.esoaring.com/albatros_presentation_esa.pdf
YouTube - Dynamic Soaring (DS) on 50ft Dam in Tennessee...
-- dam in Tennessee
-- DS at 1:51-2:35 and 4:42-5:10 the rest is slope soaring I guess.
The last link is from an on board video showing DS utilising the different wind speeds above and in the lee of a dam.
I'll redo the links in a mangled form if they disappear.
Little explanation -ette, sort of thinking out loud.
It might perhaps be thought of as picking up energy from the wind at high altitude and then recovering position in the lower wind speed at low altitude in the lee of a wave or hill. Rinse and repeat. [I just made this para up so it might be crap since I do not really follow what is going on here.] Quite how this turns into 392mph I have no idea. Ah yes, the airspeed of the aircraft downwind (300mph in a 30mph wind) is not relevant the aircraft *still* gets yet more energy out of the wind. I'm still not sure I follow myself.