Hi Phil
I live in a warm tropical climate & 28 years ago I had a thermostat fail in the closed position which cost me a head gasket on my 1300 GTJ, so ever since then I've always drilled 4 or 6 holes in any thermostat I've ever fitted. No science to it, thermostats usually have enough space to drill two or three 1/4 holes per side. Obviously when the thermostat opens as it gets warm, then normal circulation occurs. Yes it takes a bit longer to warm up, not an issue in my climate. For me the plus is that even if the thermostat fails circulation will still occur.
Evans is thick stuff, like molasses when cold, so a little slower to pass through the tiny holes when cold, but when warm the viscosity changes and it's thin like water. Like I said there are 2 camps with Evans, those for and those against, I've been very happy with it in my cars. Prior to Evans I ran Aqua-Clear.
I also run silicone brake fluid in all our classics with hydraulic brakes, never had a problem.
Ciao
Greig