I think you have it right. The TBS needle has a more agressive taper, so a shim will have a more dramatic effect than it would on the stock needle. I think you would have been running rich for sure. I'm surprised you didn't end up even backing off the main jet a little. Glad to hear it's running good. I've drilled the slide (very noticeable), but I haven't cut the spring yet.
Also, order the carb hex screw kit from Brent. It's cheap! Or, you could just grab the two different sized screws from the carb and run down to the hardware store and find them.
