Firstly, it won't change the look of the display unless you selected a font which was variable-width (such as Arial). You could use a font such as Courier New (which is fixed-width). There's another option too - Unicode provides fixed-width versions of most of the US-ASCII characters, which can be used simply by adding a constant (0xFEE0, IIRC) to the unicode character. It's probably easier just to use a fixed-width font, though, and Courier New will supposedly be as complete as Arial Unicode MS in the next Microsoft Windows/Office release.
You're right, *full* unicode fonts are big, like Arial Unicode MS. However, Arial Unicode MS is a poor example, considering it's a complete implementation of Unicode 3.2 (or so Microsoft says, anyway). Most fonts available seem to deal with more specific language groups (such as implementing european or chinese glyphs) or just the one language (i.e. an arabic or hebrew).
I'm told that the font libraries are much more efficient at dealing with these large font files these days, anyway, and the bloat is not as severe as it was originally (when the code was designed to deal with fonts of up to 65535 glyphs).
I'm also all for mirc supporting UTF-8 - I think it's vital to the progress of IRC to use UTF-8, rather than continue to squabble over a pile of incompatible character sets which have been relatively obsoleted by Unicode.
It's got to happen sometime, and hopefully mirc will recognise the CHARSET token on the 005 numeric (see
http://www.irc.org/tech_docs/005.html).