Having played about a fair bit on DA42s I have found that flying at CG fully to the rear increases cruise speed at a given power setting by about 1.5%. You could probably take that as a ball-park figure for your Aztec. Even if you did all the calculations or did some controlled flight testing, I don't reckon the end result would be all that much different.
If you fly outside of the CG limits your plane doesn't suddenly fall out of the sky (well except in extreme circumstances). All that happens is that stability goes outside of limits of what a pilot (or autopilot) is expected to cope with in normal flight, but it is nothing dramatic. But don't plan to do it deliberately under any circumstances - the slight gains in speed and efficiency you might get from a slightly rearwards CG are vastly outweighed by the grief you will get at the enquiry or inquest if you crash..