Hey Sam, I've been using a S54H off Terrigal matched to a Salty 4500H jigging for kings (PE4), its a nice rod! I generally match it up with jigs around 210 - 250 grams, my favourite pairing is is around 230 grams. Terrigal is around 100 - 110m. I occassionally drop to 150 grams, and the rod seems to work OK, I think it will be a little stiff on 100 grams, but still work OK as an infill. The 280 is meant to be a max rating, but like others have said it will handle 300 OK, so it has a pretty wide range.
I have a 3 Kings medium (overhead) and it slots in nicely below this. I have also been using a 52M (overhead) as well, which I really like. The 52M is rated to 280 (being a little shorter than the spin version) but optimally around 200 - 250 I reckon. The M class blank is considerably lighter than the the H and whilst I haven't used the S56M I could see that the 230 max rating for this rod would really pull it up, I dont think I would want to be using a 300 gram jig on that. 100 - 200 would conceivably be comfortable (I'm thinking jigging in 40 - 60m).
Hope this helps, cheers.