Well, if the action seems low for your liking, then the saddle could certainly be replaced with one at least a little higher. Getting more break angle on the strings aft of the saddle will theoretically make it sound better. How much so is hard to say, but especially with 12 strings, where half the string pins are farther away from the saddle and have an inherently shallower break angle than the other 6 strings, it's important.
I will say that I've always been a bit puzzled by the neck angles on Guild 12s. I swear that at least most of the time, Guild set 12 string necks at a shallower angle than their 6 string counterparts. I've never understood why, but I've had my hands on even new ones (though this was probably a decade ago) that had pretty marginal action with a fairly low saddle. I've seen vintage ones this way, too. The neck will be firmly attached, top will be in good shape, bridge will be "normal" but the action just isn't right; the neck angle is too low.
That aside, they're well constructed guitars, sound fantastic and having a luthier reset the neck with a greater back angle will make a dream of a guitar to play and hear. Maybe Hans can shed some light on this. Please understand I'm not attacking the brand; more than any other segment of acoustics, when I think of 12 strings, Guild seems far and away at the top. And they're still well made: the necks don't bow all to hell, tops don't cave in and warp and twist, necks don't come loose at the heel, fit and finish is great and they sound killer! It's just that the geometry seems to be wrong from the factory. The screwy thing is that it seems that the geometry is different than with the 6 strings, which usually seem right on the money. I never understood a reason why a 12 string should play with higher action than a 6 string.
There was once a time when I'd have said that Guild didn't do this with their SF-XIIs, as my friends and musical mentors Scott and David had a pair of early 70s, large pickup SF-XIIs that played just perfect. Ridiculously easy to play 12s... Then I got mine... It's one of the first SF-XIIs made, a '66 with the "short neck" and smaller pickups. It's just like a lot of the acoustics I've played. It's all together, nothing is warped or caving in, neck joint is totally solid, but the action is just marginal. In fact, it's got a factory floating Hagstrom bridge, instead of the usual rosewood topped unit and I don't think the action would be playable at all otherwise, since the Hagstrom top piece will go a lot lower than the taller rosewood top. Maybe the consciously set the neck at a shallow angle in anticipation of the lower Hagstrom bridge and just over did it?
I'd say that, with an electric, the geometry should be such that, if you adjust the bridge all the way down, the strings should be touching, or almost touching the fretboard. I'd say with an acoustic, the same should probably be true. the top of the bridge should be "about" in line with the plane of the fretboard, such that if you sanded the saddle down flush with the bridge, the strings would be almost sitting on the fretboard. With that sort of geometry, you should end up with good easy-playing action with a nice medium/tall saddle, which will sound the best, too.