Your question is a bit vauge, do you mean for debuffs or buffs?
Tier is usually reserved for buffs, in which case most buffs are easily capped at 75 with no skill enhancements. I believe Minuet IV requires a total of 462 combined wind/sing skill to cap. After that, Victory March increase in intervals of 1/1024 (ie: +0.097% haste) over the course of several skill levels. There is no known cap to march, other than the fact that we are limited by how much skill we can actually equip (meaning, if SE adds more skill it may or may not keep going up). I believe the last March tier requires every bit of skill gear obtainable in game for each slot.
If your talking about tiers for debuffs, there is only 1 song that actually has tiers and it is already pre-capped at 75 - Requiem. All other debuff songs do not scale at all in relation to skill. If you want a debuff song to become more potent, use the appropriate instruments.
Or are you instead talking about Magical Accuracy (Hitrate)?
Since you specified HNM I will cover that portion of it, keep in mind this doesn't mean it applies to all mobs (below is overkill for 'normal' mobs).
Most common HNM's (Sky, Sea, Kings, etc) seem to have their base statistics centered around 120 (ToAU+WotG seems higher). This applies to STR, INT, MND, and CHR at the very least, and possibly to all stats. This also somewhat explains the magical '120' stat that is seen across various jobs when talking about anything from defensive physical builds to offensive magical accuracy. While the 120 stat was always considered a rule of thumb, not much testing has been done to prove anything until recently (see link
). With that said, there is a relationship between stats, skill and magic accuracy gear that is somewhat dependant upon certain conditions.
If you read the article and the information behind it, you'll realize that your ideal set is going to consist of approximately 120 CHR for HNM fights. You can push yourself a bit over that (no more than 130 CHR maybe), but keep in mind that we can always eat food or sing CHR songs to further buff ourselves if needed. After that, the benefits of adding more CHR diminishes while Skill and Magical Accuracy gear remains consistant. So far, there does not seem to be anything pointing to a diminishing point for Skill and Magical Accuracy gear - Meaning, you can add as much as you want.
Keep in mind that the articles don't tell you anything about your overall magical hitrate, nor do they tell you how to calculate it. There is also a level difference function that will be the *Main* obstacle to overcome regarding magic hitrate. It only tells you how much you can expect your magical hitrate to increase by on a particular mob (ie: 1 skill increases by
1%). This goes without saying, if your landing rate is low to begin with, the gains you will recieve will also be low, and vice versa. Stacking on more will have a cumulative effect over your previous gains (think of it as intrest) and, with enough CHR, Skill, and Magic Accuracy gear, you may be able to (slowly) overcome resist rates.
With that said, there is no limitation the amount of skill you can add for debuffs.