Yes, you can test against any URL that you can reach from your computer that is running Visual Studio. You simply need to parameterize the host url(s) in your webtest(s). Click the Parameterize Web Servers
button in the web test editor toolbar to achieve this. Once parameterized, you can substitute different values by creating the same context parameters in your LoadTest RunSetting. LoadTest context parameters override WebTest context parameters.
Of course, you are still subject to whatever network obstacles and firewall rules that are in your way. For example, you won't be able to hook up to capture Performance Monitor data from the Amazon Cloud servers (or at least I presume that the relevant ports would be blocked).