The easiest way to ask a good question is to have it about *a real problem*. Lets pick this question apart.

From your question

> "This hypothetical setup would be a vanilla Debian installation with
> nginx and such."


From the [\[FAQ\]][1] 

> you are asking an open-ended, hypothetical question: “What if ______
> happened?”

Also from your question
> How does this setup compare between a Raspberry Pi (maybe a couple of
> them for balancing), a desktop used for everyday tasks, and a
> commodity rack mounted server?

What sort of desktop? What sort of loads? Where do we reasonably expect a bottleneck ?

 Would we be comparing a modern core i7, a still reasonably competent and realistically retired c2d?

What sort of rackmount server? A brand new 4u server running dual xeons and inconceivably huge amounts of ram?  someone's old alpha we pulled out of a university dumpster>

In short, you've asked a hypothetical questions about hypothetical hardware setups, and expect it to be treated as a real question. 

The 35 dollar + S&H question I'd ask is *why not try it?*. Its one hell of a geek toy and cheap enough to buy after mowing a lawn or two. Throw on a copy of raspian, try your setup, and see if its good enough for your needs. 


  [1]: https://superuser.com/help/dont-ask