. . .So, was the Sao Paulo better, worse or similar to the original?
What had O'Brien changed? Had he solved the original problems, or had he merely imposed flight envelope limitations so that the problems wouldn't manifest? Had he upgunned or downgunned the ship, upengined or downengined, tuned or detuned? Was the Sao Paulo a series production model, or simply the last of the original, failed and thus expendable prototypes?
I always assumed that information about any upgrades O'Brien made to Defiant
's systems were forwarded to Starfleet, so that other ships of its class could be modified. This would also to apply to Scott, LaForge, Torres, and Tucker in terms of their ships (though Torres is the exception, until contact was re-established with Alpha Quadrant).