VGs sure helped my Heavy Hauler wings. First installed under horizontal stab, yep good improvement there. Later did the wing, no numbers but at my 1000' strip with trees around the pucker factor was definitely improved.
Just reviewing a bit and feel I should respond to the portion of your reply that I underlined. Beware, cold seizures definitely do happen on these engines (gray head or blue head) with however many thermostats one might or might not have! Congrats on securing the airport spot!
John, I didn't disagree with you on the primary design purpose of the hole. I'm glad you know the coolant temperature differential spec, I wish you well in achieving it and cabin heat. Let us know how it works out for you for the benefit of all of us and those who follow. Don't forget the sage advise also regarding holding accessories out at shoulder height and letting go of them to see if they go up or down. Blue skies and happy flying.
Jackson Ski Plane 2018 McGregor, MN. Those rear skis are slowly getting closer to becoming Avid skis. I'll add that the Jabiru heat muff while being modest in the Avid is definitely better than the heat in the trike was.
The hole also lets a small amount of coolant to flow (my factory original hole was smaller iirc). I no longer have all my Rotax manuals but I recall a minimum coolant return to engine temperature specification. Give it serious respect. In the picture below you can barely see behind my head a pink 2" wide velcro strap wrapped around my radiator. I had two of these I'd install depending on ambient temp as needed to keep radiator temp up. My heater left a little to be desired when below 0F, no carbonmonoxide worries though.
I believe you are setting yourself up for a cold seizure. You need to block off the front of the radiator like Lenny says and forget about shutting off flow to it with a separate thermostat. If the radiator liquid gets too cold, then when you go for more power the thermostat will sense heat at the head and open. When it opens it will allow for a cold slug of coolant from your radiator to hit the bottom of the engine and there is your cold seizure. And yes, you do want the 160 degree thermostat for sure.