Most 64-bit benchmarks show that 64bit applications only have speed benefits when dealing with large amounts of data-- we're talking over 2gb. For small data, which would be the overwhelming case for an IRC client, the performance boost is basically non existent, and I've seen benchmarks where the overhead of 64bit was actually a detriment (though not really significant) to speed. This is to say that there is little evidence to support the "speed advantage" assumption.
