Let’s test its strength.
Say they reduced it to 100 MP. Stacking it to max alone would cost 300 MP, but let’s consider the returns:
On a 75 WIS Pally, the first use of the seal will last 7.5 seconds, the second use would be 9 seconds, the third would be 11.5 seconds, totaling a net cost of 269.5 MP, discounted over a course of 12 seconds.
Boring Math
For the first 0.5 seconds, you have +15 WIS (+0.9 MP regained);
the next 0.5 seconds, you have +22 WIS (+1.32 MP);
the next 6.5 seconds you have +25 WIS (+19.5 MP);
the next 2 seconds, you have +22 WIS again (+5.28 MP);
and for the last 2.5 seconds, you have +15 WIS again (+4.5 MP),
Next would be to calculate the effects of WisMod on various abilities within that 6.5-second window:
Assuming the characters are just below the threshold for WisMod (which are very low for some classes)
Durations and Ranges
Most abilities with duration will last 2.5 seconds longer. +50% duration for 5-second long abilities. This affects tiered Tomes, Orbs, and Seals. Helms last 4.5 seconds, but close enough.
Relevant UT/STs with +2.5 sec durations are: Soul of the Bearer, Helm of the Juggernaut, Tome of Holy Protection, Book of Geb, Sandstone Seal, and Seal of Cubic Conundra.
Oreo lasts 0.5 seconds longer.
Most abilities with range will have an extra +2.5 tiles of range. This includes all tiered Tomes, Orbs, Seals, and Helms.
Relevant UT/STs with +2.5 tile range are: Tome of Pain, Seal of Cubic Conundra, Enchantment Orb, and Orb of Sweet Demise.
Skulls gain +1 tile healing range but do not gain a wider damage radius.
Scepter of Fulmination gains 4 more targets, +80%. You can reach a whopping 10 targets on a maxed Sorc.
T6 Scepters, Cnidaria Rod, gain 2 more targets, +40%.
Scepter of Devastation gains 1 more target, double! The same goes for the Grotesque Scepter, missing 2 WIS for an extra target.
Heal, HP, and Damage
T6 Tomes heal 75 more HP, +47% to base;
T6 Seals add 50 extra Max HP, +71% to base;
T6 Scepters deal 200 extra damage on their first hit, +66% to base;
Scepter of Devastation deals 250 extra damage, +30% to base;
Cnidaria Rod deals 125 extra damage, +83% base damage to the first target;
Grotesque Scepter deals 150 extra damage, +16% to base;
T6 Skulls will gain +100 damage, +56% to base;
Tome of Pain deals 175 extra damage, +35% to base;
Note about the WisMod Formulas
I’m using the WisMod formulas provided on each item’s page in the wiki. Some values I’m getting from the individual pages don’t seem to correspond with their descriptions in the ability pages.
Tomes for example:
This formula results in 235 HP yet in the Tomes page this appears
I believe the general ability pages are merely outdated.
WisMod classes tend to have +10 to +25 WIS above the threshold at max anyway, so percentage values may matter less than the raw boosts.
To me, a cost of 269.5 MP for a +25 WIS buff that lasts about 6.5 seconds still seems rather high, considering spamming the ability all three times requires you to really wait things out if you have an MP boosting ring or to have a very good pet to even perform.
Of course, to make things a lot simpler, DECA could simply give the item a buff to its Wisdom boost to +20 or +25 and introduce a cooldown, as you suggested. Even then, I’d still lower the cost to about 120 MP. That way, you can still use another Seal once.