JayDDee,
Thanks for your comments, I'd leave it at that for now as many others would be reading this thread.
Rather, it is correct to say that -ftree-vectorize is not a 'miracle pill', but that in some cases, one may observe an edge, and actually it isn't that much more, I think that 20% gain is not rigorously measured.
i.e. expect less gains in fact with the use of -ftree-vectorize. But that given my experiments, it is likely there is 'some' gains, but not a lot.