I think you should try more timing. You should be pushing as much timing as possible without detination (pinging). 15 already sounds like a good amount but I've heard of stangs running good with up to 18 and 19 degrees. Test and tune and a FPR is vital.
Last but not least, vacuum leaks, see if you can hook a vacuum gauge on and let us know what it reads at idle and how it performs when under load.
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