Firstly, there's no official way to add the iOS 4.3 simulator. You may be able to hack it somehow but I don't know how and I wouldn't recommend it.
Secondly, if you really want to support iOS 4.3 you need an actual device to test it on. The Simulator is not 100% accurate and it certainly doesn't mirror the performance of a real device. If it existed, using the Simulator would be a good "first pass" but it's not a substitute for the real thing.