I just realized again that mIRC is my only communications application that doesn't have a built-in spell checker.

I'm so OCD about spelling things correctly, because IRC trolls are so OCD about picking out people's spelling errors, it really cuts into my time when I have to open a browser window just to google the correct spelling of a word.

This really sucks for someone who types 120 WPM. Very hampering in the exchange of information.

Well that sounds painful.
I'd use a script or at least something like tinyspell to speed things up.
I always wish these things were mentioned in the help file.

Try Dragon Naturally Speaking. As I've gotten older, I find my eyesight failing a little, and I don't type that fast. I've used it for several months, now.