Depends on how the D is fitted. If they haven't retrofitted or added any tech they've pillaged then it'll be like the fight in Yesterdays Enterprise so maybe like 5 Klingon Birds of Prey would be able to take them out. IF the ship has been fitted with stuff they've stolen from the Romulans or other places then the Enterprise could be something like the one in future timeline of All Good Things where they could easily spank the Klingons and Romulans.
If they're pillaging along the Neutral Zone than chances are they've now got a cloaking device. They've also probably added disrupters to the ship. Plus if they're selling whatever they're stealing to say like the Farangi or other races they would have likely gotten tech from all over the place.
So a standard Starfleet fitted Galaxy Class? not long, it's still technically a science and exploration vessel. an upgraded Galaxy with tech from several different races? could potentially make it the most powerful ship in the Alpha and Beta quadrant.
ya don't say