Joseph, the second above throttle plate Groove was "discovered" by Sergei a Gadgetman in Russia who had the Certified level bit set like I do. There are special bits in that set, some are what Ron called reverse bits. Reverse bits like it implies cut a Groove in the reverse shape of normal. You could try to do it with a regular small bit, but I can't advise it. It would be real "Interesting" experience IMHO. If you use a small reg. bit held against the throttle plate from above, the Groove would be backwards, remember it is a directional design. I'd concentrate on ironing out what you have right now first. IF ya want to mess around w/ another TB that's the same correct one for your ride, you could, but I wouldnt try on the only TB you have for a daily driver.
The hard starting in the Wisconsin cold- is it that it's just plain hard to get to start, or that it won't hold a fast idle when cold? I'm going to guess that the fast idle IAC is controlled by coolant temps. Climate is a consideration when doing the IAC mods- Ron was in Phoenix for most of his mods, never had to sweat cold weather, hot-- oh Yeah!
In many /most cases the ECU uses the IAC to increase idle RPM when A/C is engaged. IAC flow path can be restricted to enhance man. vac., but the climate and usage need considered when figuring how much restriction. IAC is an analogy to both the old carb. fast idle cam, AND the A/C throttle positioner solonoids from the carb. days.
Did enlarging the IAC restriction help?
Tracy G