?????

I'm unsure if that would be faster really. Since these trees are generated on-the-fly without rebalancing, it's not like they're hugely optimal in the first place. I doubt the overhead of changing the weighting would be much compared to how non-optimal it is in the first place. Additionally two trees could be worse from the perspective of on-CPU caches. IMO it's worth trying both ways and benchmarking.

If balancing the tree means better performance, would the end of a round (after death or victory) be a good time to balance the tree, or would it even make a difference in terms of performance during the match? I suppose it all depends on how hard one's bot is pushing up against the edge of the time slice it's allowed.

The end of the round probably would be a good time yes, though I've never really tried it. The re-balancing would take longer each round though, so it may make sense to only do that for the first few rounds of battle. As the number of rounds increases there would be diminishing returns to constantly re-balancing anyway.