I'm going to say that it wouldn't be helpful in most cases. Let's consider the different input methods associated with form input - your assumption is that an email address is manually typed out. Starting with that, I think there's a little bit of overhead involved with autocompletion.
For example, if my address is firstname.lastname@example.org, I'm going to begin typing
nic@company., at which point I'd probably get my dropdown options for autocompletion. Based on the quantity of different TLDs, that could be more than a few. Think about
.co.au, and more - processing that list to find what I want seems like it would take longer than typing the 5 extra characters.
Additionally, email fields on mobile usually get some assistance from the keyboard, and this whole conversation is moot if autofill is used (which, by the way, would be good to know). Is it worth testing? Possibly, but I speculate that there won't be many advantages.