I believe eggdrop is the most active/popular bot and it's written in tcl ( and according to wiki the core is C but I haven't confirmed that ).
I'm wondering if there would be any performance benefit of recoding it's functionality in node.js or Python, in addition to making it more accessible since Python and JS are arguably more popular languages and not many are familiar with tcl.
So, how would they stack up vs tcl in general, performance-wise?
As you suspected, eggdrop is not written in tcl, it is written in C, however it does use tcl as its scripting/extension language.
I would expect that in the case of an eggdrop, the performance difference between using tcl as a scripting language, and using Python, Lua, JS, or virtually anything else would be negligible, as eggdrops generally aren't performing high load tasks.
In the event it really was an issue, your question would need more specifics. Performance for what task under what conditions? Memory use? CPU efficiency? Latency? And the answer would probably be "measure and find out". Given the typical use of an eggdrop, it doesn't take particularly efficient code to respond to the occasional IRC trigger command once every few minutes or hours.
As a more general case, I'm sure you could find benchmark comparisons of specific algorithms or tasks performed by various scripting languages on particular operating systems or environments, at which point it wouldn't really have anything to do with IRC or eggdrop.
If you're not doing much other than waiting on a quiet channel for something to happen, performance is pretty much irrelevant. You could probably write that in BF (well, with network connectivity primitives added) and have it perform OK.
If you're running on lots of busy channels with many things being watched for, that's different. Tcl's very good at event-driven IO, which is ideal for this sort of situation. (Python can do that, but needs external libraries, as does Lua. I don't know JS enough to comment there.)
If you're needing to do significant non-IO-bound processing for some message responses, you're into needing threads. I know that both Tcl and Python support threads, but with utterly different threading models (Python has a shared-memory model which makes it easier to pass some types of task around, especially when the data is large, and Tcl has an apartment model which greatly reduces the amount of locking required in the implementation for a good performance boost in CPU-bound code).
How is that relevant for IRC bots? Well, it all depends on what you're doing in the bot.