The only real limitation of Tier 1 is the inabillity to extend the language.
In Tier 2: Need XML? Need UDP? Need databaseconnection? Need to connect to an USB device? All those things exists in forms of C++ libraries.
In Tier 1, you'll have to do it with the commands that exists, and if something is impossible (connect to a USB device): You just have to wait for Paul to (maybe) implement it. Or, I don't know, send them a bunch of cash or something like that. Some things, like XML is entirely possible to do on your own (or someone from the community) because it are just basic operations that are possible within AGK.
BUT: With the announcement of the possibilty to use shared libraries (DLL on Win, so on Linux, don't know on Mac) my post may become obsolete.
As Easter Bunny says, speed is a very bad argument pro C++. While it's theoretically true, it just makes no sense these days.
Also, all heavy duty graphical processing is done via commands, which are written in.. C++. CPU tasks will be indeed slower, but again, that's theory.
Unity uses JS or C#, never heard anyone complaining about speed.