Firstly, there is no official way to add the iOS 4.3 simulator. You can hack it somehow, but I don’t know, as I don’t recommend it.
Secondly, if you really want to support iOS 4.3, you need to really check it out. The simulator is not 100% accurate and, of course, does not reflect the performance of a real device. If it existed, using the Simulator would be a good “first pass”, but it would not replace the real thing.