I just took temperature measurements while driving around, with my nifty Radio Shack indoor/outdoor digital thermometer.

The engine was already very warm from previous driving:

'Inside' Heat shield
Max - 81.0º
Min - 73.4º
Avg - 75.6º

Inside the car (1 window open)
Max - 78.3º (blazing! )
Min - 75.9º

And finally, on the other side of the heat shield:
118.2º

So I guess it is working...at least partially...