I think the subdomain is a good idea for different locations, but I would do that on a .com domain. So have a .com domain and the use subdomains for say your usa and aussie etc sections. Obviously a .co.uk can rank in the USA but you are putting yourself at a serious disadvantage. I am not guessing here as I have read around this subject in the past when moving my own sites to different domains and google states to use .co.uk for UK, they do not suggest that a .co.uk is good for general global use. Also if you are moving site then at least do it to a .com, why go to all the hassle for an extension that may not be factored as highly as a .com in the algorithm. I really don't believe all TLDs have a level playing field.