I tried this a couple of year back in the winter, I made piece of cardboard to fit in front of the radiator, covered it in parcel tape to keep it dry. It completly blocked the airflow to the radiator. It made NO DIFFERENCE at all. I could not work out why at first, then the penny dropped, until the thermostat opens there is no water circulating through the radiator, only through the cabin heater matrix, this is so effiecient that it alone was cooling the engine helped by the near zero deg C outside temperatures.
I think this type of thing worked back in the 60's/70's/80's because the thermostat allowed some water to circulate through the radiator, my dad alsways blanked of 2/3rd of the rad on his car in the winter
On another note I changed the thermostat in my TDI75 from a Euro car parts circoili one that I fitted a couple of year ago to a Gates 87deg C one that I bought a couple of weeks ago. The hope was that the ECP one was not great quality and was opening too soon. Well, with the help of my worx 1/4" drive socket and small ratchet BOTH bolts were easily removed without taking the alternator off (this makes the job soooooooo much easier) and the gates stat fitted, coolant replenished and air blead out. Proof was this mornings commute to work 7.5 deg C on the outside temp so Webasto did not cut it, just moving of 60 deg at the 5 mile mark, cabin heater set to hot, but only blowing look warm.
Difference to the ECP stat is NIL
This bears out my earlier (2 years ago) observation that the ECP stat and the original Audi stat I removed where identical other than the VAG logo, tool marks and crimps all in exatly the same place as tha Audi stat.
Only save in grace was the thermostat swap was finished in under 30 mins start to finish and most of that was getting the air out of the cooling system
Ha well the heated seats are a God send in this cold weather, heated cabins are for wimps
Paul