Chang was playing with Kirk anyway - one shot to the bridge or to Engineering could have finished Kirk or the A once the shields were weakened. 'Excelsior's been hit' seemed to be more concerning, they bought down the bulkheads and everything.
At least the Duras sisters were planning to do the sensible thing, target the Bridge and finish it, they just left it too late.
Even the battle of Wolf 359, you have to wonder why the Borg didn't just take out the bridge of each Starship and then assimilate everyone, tremendous waste of resources really.
As you say, in both TUC and GEN, it's pretty obvious that the Klingons were toying with our heroes, and they wanted our heroes to know that. Shooting at the bridge or warp core while the Enterprises
were vulnerable would have destroyed them altogether.
If we assume that the Duras had some level of competency, had an effectively unshielded Enterprise
-D in their sights, and still wanted to toy with Riker and company, the Duras would have probably scanned the Enterprise
for prime targets. I would guess that they would have shot out the forward torpedo launcher and the power conduits feeding the forward phaser arrays. This might explain why the Enterprise
tried to turn around rather than stand it's ground.
As for how powerful a Galaxy
-class starship is, there is an actual canonical source
The Best of Both Worlds wrote:
BORG: Captain Jean-Luc Picard, you lead the strongest ship of the Federation Starfleet. You speak for your people.
Now, it's uncertain whether that statement can be used as a blanket statement for all Galaxy
-class starships or if the Enterprise
-D, having quite a few combat missions in addition to simply exploring, has a configuration that indeed makes it the strongest (the "Federation's flagship vessel", if you will).
It's safe to say that Galaxy
-class starships, at least in early 2367, are at the very top of the ladder in terms of power, although there seems to be a range depending on configuration. (Jellico, for example, thinks the Enterprise
-D could be even more combat-ready and gets Engineering to make the changes.)