Photons are also not visible. Beams are visible.
Conclusion -> Beams do not consist of photons. They consist of some charged particles that emit photons around them.
If the beams were true photon beams, you could only see bright light at the hit point and probably some vaporizing metal, and possibly some diffraction/glimmering from gas and dust particles on the beam's way through the battle area, but not the solid light brite show we have in FS2.
That's the physical approach anyway, in the same sense that the argument of shields not stopping photons relies on physics...
I know they are called Photon beam cannons in the first mission of FS2 Campaign, but physically it is not "possible" that they would be just photons.
That said, the match between ISD and Orion really depends of how someone wants to balance the ships. It's just up to whichever is preferredf to win by the mission designer. If someone thinks Star Wars shields can't stop beams, that's a valid approach. If someone thinks Star Wars shields do stop beams, there's no evidence to the contrary either because there's no canon interaction between the universes (thank the Flying Spaghetti Monster for small things).
IMHO, it's probable that SW shields would hold against beams since they do hold against turbolaser shots, which are essentially much closer to pulsed beams than FS2 turret blobs... If you look at the New Hope trench run, the TIE fighter shots look more like Minbari Nial's neutron beams (fast, repeating lances of light) than the relatively slow flying neon tubes from X-Wing series of games.