well to compile it for use within mIRC itself certainly (I think i even mention it) would be something to see. I just take it from his post saying exe that he meant an application used "outside" of mIRC.
Anything that would speed up the language im all for at least to an extent.
Placing script within a dll wouldnt be hard but it would take quite a bit of work. If you wanted to you could take advantage of the mapped file and do it that way but you wont gain anything in the way of speed because mIRC will still have to parse any commands you send it. The only advantage would be maybe "hiding" your script code within a dll. somehow i dont think that would go over too well.