Sorry, I missed these replies. Thanks for the replies.
it seems a bit odd that my turbo versus the next one up have the same frame / housing as in they are the same outside dimensions but my 7064 would run high back pressure but from your replies I guess it means my turbo would be struggling to supply enough air to feed the strokes Engine at say 2 bar whereas the "bigger" turbo could flow more and therefore have less back pressure.