Yup, tried my car racing set under my leathers as its 36C here in the daytimes at the moment and it makes a huge difference to feeling of pain from the hot leather near your skin. I have the Alpinestars stuff which is the lightest full FIA spec stuff, not cheap but does the job for two things now so finally getting more value for money!
It does mean your bits get a little more sweaty though!
Makes riding possible in proper gear though.
Oh and I have full termi with the new heat shield but not the engine covers.