The extra 2gb is to manage the large 1tb of storage and that is all. Speed test have already proven that the IPP with 1TB with 6gb of ram is actually slightly slower than the IPP 512gb with 4gb of ram. There is a video of these test on YouTube. You are not getting any usable performance gains by getting the 1TB model other than massive storage.
Go to 2:20
Go to 2:20