I ran 26-28 PSI on a T-64 for a couple years on Sunoco GT Plus (99 MON, 104 R+M/2) and then when I pulled the head to port it there was NO sign of detonation whatsoever. Then a couple years later I did a compression test and the numbers were still in the 170 PSI range across all cylinders (very upper end of the specifications). I say all this to preface my belief that you can run that kind of boost on the factory ECU with a good unleaded fuel.
As far as what power, I later made 682 whp (Dynojet) on ~24 PSI (on an SP74 with headwork and 264s, and some mix of unleaded and leaded fuel).