I was thinking about "Yesterday's Enterprise" recently. I know that despite the time alteration, they had to keep a lot of things on the Enterprise similar for the sake of familiarity to the audience. But there's no way that the Galaxy-class ship as we know it would exist in a timeline where the Federation has been at war for over 20 years. They would have ditched the sleek luxury cruiser in favor of a massive gunship that would let the Klingons know that they weren't fucking around. Also, there definitely wouldn't be a bar and lounge on the ship, but you need to have Guinan on board to notice the time distortion. Maybe she's a military advisor or something.
"Yesterday's Enterprise" (TNG Season 3) was set in 2366. According to the TNG Tech Manual, the Galaxy class design program started in 2343, with actual construction of the Enterprise (second in production) starting in 2353. It's easy enough to imagine that much of the design work was already well underway when the alt-timeline Klingon war started, and the overall spaceframe design was carried forward.
Even in the main timeline, much of the internal volume of the Galaxy class ships were essentially unfinished (again per the TNG Tech Manual) to support future expansion. In DS9, showrunners have speculated that the War Galaxy variants mass produced to take on the Dominion were even more spartan, with skeleton crews and only tactical and power systems fully built out.
In such a war it's very likely that many of the internal systems would have been focused on weapons and shields from the get-go. The massive Galaxies would also serve as excellent assault and troop transport ships with minor modifications (am I remembering correctly that there's even some mention of onboard troops in the episode?). The ship would also need (a bigger) ten-forward, more-so than they did in the prime timeline.
Galaxy class even it in their peace time "luxury cruiser" guise was still one of the most heavily armed and capable ship classes in the (Beta and Alpha) Quadrant. It wouldn't be until the Enterprise-D's encounter with the Borg. Then later the Odyssey's destruction by the Dominion was the Galaxy's place as top dog was shaken. As seen in DS9, (War) Galaxies are very capable of taking the fight to the enemy. If there is one weakness, it is the Galaxy using the older modulation system of using swipable internal modules. Instead of the newer external module system i.e. mission pods that is used with the Nebula and newer ship classes.
This is correct, although the Galaxy class had a bad track record in peacetime, indicating some significant problems with the design. The Yamato was lost due to lack of anti-virus software. The Enterprise through poor security and shield issues. The Odyssey lost due to sheer fucking incompetence by her Captain. We also know the Enterprise had at least a dozen near warp-core breaches over it's short service life, showing serious flaws.
However, they seemed to be very capable in combat. We never saw one lost in any of DS9's massive fleet combat scenes, even when Excelsiors, D'deridex warbirds, and Vor'cha assault cruisers were exploding left and right. Starfleet was fielding the tactically oriented anti-Borg ships at that point, but the Galaxy wings remained the anvil that allied fleets used when hammering Dominion and Cardassian fleets. It's heavily implied that they remained among the most survivable combat ships around, even if they were no longer offensively the most powerful.