Yes, there is a cap.
I think it's a slight misconception that a players contribution is solely based on their threat, but this doesn't seem to be true.
In off-peak times with very little people hunting you can easily achieve full credit from a mark by doing very little of anything. But when it gets busy with hundreds of people attending getting full credit (on S ranks particularly) becomes extremely difficult even if you perform at your absolute peak of maximum threat generating efficiency. Why is this? It's because the number of players that are able to contribute to a single mark is LIMITED. Only the top x00 players on the global threat list will have their contribution to the mark count when it finally dies.
You will notice in busy periods that many members of your party will not have a threat bar next to their name. Or they might have one at the start but it will disappear soon after the fight begins. This is because that person is being knocked out of the top x00 list of contributors. Even if your group is on the threat list of an S rank for 90% of the fight and at the last 10% you are all knocked off... No credit.
TLDR; You need threat to remain a contributor to the fight, but as long as you are on the top x00 threat list you can pretty much auto-attack a mark to death and receive full credit.
General tips;
- Establish yourself on the mobs limited threat list IMMEDIATELY after you see it go Purple/Red (tag it with a damage dealing skill). Even a 2 second delay on a busy S rank will count you out of that fight permanently.
- Instantly follow up the tag with your highest threat generating skills to ensure you don't get knocked off the threat list. Again, if you fall off that list you will almost never get back and all your contribution is ignored.
- After the tag and initial massive threat dump continue with your most consistent high threat skills until the mob is dead.
- Tanks: Sword Oath is better threat via DPS than Shield Oath is.
- WHMs: Precast Medica II and Regen. Stop precasting anything when you feel a pull is imminent. You cant be sitting in a GCD when that mob turns purple. Aero > Benediction (on the highest HP party member) > Swiftcast > Divine Seal > Medica I > Presence of Mind > Medica I spam (or Cure III if your party is adequately stacked) is the best combo I can come up with.. I would advice against using Shroud of Saints at all but try and precast it when you feel a pull is imminent. If you are attending a busy S rank never use shroud of saints after you have started generating threat, even if you hit 0 mp.
- DPS: You are generally screwed on peak time S ranks. Sorry.
- SMN and SCH are almost 100% worthless even in only slightly busy periods.
- In my personal experience, tanks are kings of consistently getting on and staying on the contributors list. WHMs are able to generate massive threat (way more than tanks) but seem to have a higher risk of getting knocked off the contributor list early on in the fight before they can start spamming.