The Excalibur was larger than all but the most massive Vorlon and Shadow ships, not including Planet Killers. As such I imagine its main guns packed more of a wallop. With Vorlon self-repairing and energy refracting hull technology, it was obviously capable of resisting Vorlon weaponry, but Shadow weaponry included plasma blasts, which it would still be susceptible to. Remember that both Minbari and Vorlon ships were susceptible to nukes and projectiles, so any ship constructed using their technology should still suffer from such weapons.
All-in-all, with a suitable power source removing the weakness of the main guns, the Excalibur was more powerful than either Vorlon or Shadow vessels, and likely more durable because of this. But it was also a lot less manouevrable than either species' ships. All-in-all, I don't think the Excalibur could take a Vorlon ship of comparable calibre, and it could only match up to a Shadow vessel if it had a telepath on board. The telepath wouldn't need to hold the Shadows for long though, as the main guns could take care of a Battle Crab even faster than a Vorlon Cruiser.
I did, and I make women buy their own drinks. Totally worth it, unlike most DVDs.
