Unfortunately copy of the skin of Gmail would not get you the gigantic honking datacenters that make Gmail so much faster than Mail.app. Would you pay for a client that looked like Gmail and performed like Mail.app?
The only thing I can think of where honking datacenters could conceivably help is searching for things. All other things should be mostly unaffected. And I'm not sure you need an entire datacenter to search through one person's email or address book.
Spam protection is a more serious issue, I think. Both mail servers and desktop clients do it. Mail servers, especially if they get feedback from clients, have a huge advantage though in that they can learn from all their users. The desktop client has to work with a single user's input; unless you implemented some kind of p2p collaborative approach (which would be really cool).
The day-to-day performance of Mail.app is orders of magnitude slower than Gmail. Loading a thread with 100 replies in Mail will take several seconds. The same thread in Gmail will load in milliseconds.
Yes, but that's just evidence that Mail.app is badly optimized, for that use case anyway. You don't need a server farm to get a couple of hundred replies to display quickly.
