In regards to the design of the Defiant...it did blow shit up real nice, but...ahem....got beat up real nice by the borg in First Contact...the very thing it was intended to defend against.
The Battle of Sector 001 was the first time that Starfleet had gotten the chance to engage in combat against the Borg since the Battle of Wolf 359. Wolf 359 led to all but one of the starships that confronted the Borg being destroyed; Sector 001, by contrast, was apparently a running battle between the cube and Starfleet that lasted several days (since we know the Romulan Neutral Zone, where the Enterprise was, is several days from Earth in 2373) and stretched across Federation space. And we know that the Defiant was there at the beginning of the battle, and that she survived until the very end. Sounds to me like the Defiant, and the rest of Starfleet, did very well and strongly improved their track-record.