Anyway, if you don't mind patching mirc.exe and using my "solution", you could use gzip encoding. Which version are you using?
I'm using 6.35. Somehow I have my doubts about patching though, I'm somewhat afraid it will break things.
It's up to you. The modification only changes 2 bytes. And it doesn't target anything else.
The last solution, if the server response can't be fixed for "deflate" requests, it to remove the "Accept-Encoding:" header from your requests.