I was reviewing some N1 Grammar when I noticed that both of these seem to translate to "when it comes to", "as for" Is there any difference between the two in terms of nuances? I'd really appreciate it if you can cite sources.
Xに至っては has the nuance of "moreover", "on top of that", "what's worse/more", etc. It means X is even more extraordinary than what was already mentioned. The examples in the linked page lack the context, but 美智子に至ってはうきうきしているようだった makes little sense without a proper previous context, like:
Xに至っては also means "now that we have reached X", "(we're already) at this point", etc. 「ここに至っては、もう祈ることしかできない。」
Xときたら is just a colloquial topic marker, and the predicate is usually something bad. It's typically used to speak ill of something/someone, as shown in the examples in the link.