Notice the "chains" after some of the timestamps. They indicate that the messages arrived within 0,5 seconds of each others, and that their ordering most likely isn't important. See the demo in the screenshot after raksi joins. If you read the messages sometimes later without the chains, you might wonder why raksi is asking the URL even though it was just given.
Isn't it annoying when you're reading scrollback that you can't see what people are currently talking about? Not a problem with irssi2! Also notice the built-in trackbar.
Right-justified timestamps would be possible, although they'll probably mess up copy&pasting.. It could still be modified so that the timestamp is written only when the minute changes.
Mockup of what an "overview" window could look like.