Ok... I did some testing in order to go a bit in-depth on the intellect stat. I'm sure the people who like playing a mage hero will rub their hands in expectation.
So, here's what can be achieved theoretically:
Forenote: There is a possibility of spawning 2 Apprentice's Dress. I had this happen in at least 2 of my playthroughs, so I can say this for a fact. So having 2 magess' dresses is completely plausible and normal.
Own equipment:
Helm - BattleMage's Helm - +1def/+3 INT
Weapon - Archmage's Staff - +5INT/+20% Lightning Damage (spell and archmage attack)
Armor/Dress - Magess' Dress - +4INT/+800 Leadership/+30% Archmagi crit rate
Boots - Mystic's Boots - +2INT/Improved slow/haste spells
Regalia/Artifact - Royal Snake Ring - +5INT/+3 damage to all snakes in your army
Shield/Artifact - Moneylender's Ring - +5INT/-30% gold in battle
Belt/Gloves - Well of mana belt - +3 Mana/Round, +20% mana regen rate
Artifact - Ancient Amulet - +4INT/+15 mana (10 magic runes after 50 victories)
Total: 28
Elenhel companion: (+4INT bonus)
Shield/Gloves - Scaly Shield - +2def/+3INT
Dress - Magess' Dress - +4INT/+800 Leadership/+30% Archmagi crit rate
Belt/Regalia - AntiMage's Diploma - +2def/+2INT/+25% magic resist (absolute)
Artifact - Dead Skull - +5INT/+5 Atk
Total: 18
Skills:
Language lore - +6INT
Dissertation - +6INT
Scouting - +3INT
Learning - +3INT
Total: 18
Additional:
Ring of Intellect - +3INT (permanent after 30 battles)
EDIT: Additional 2: Illiteracy Exterminator medal - +3 intellect
Grand total: 28+18+18+3=67 + 3 =70
That is before bonuses from game objects and altars that add +1 INT. So generally, a mage hero can have over 70 INT.
70 INT transforms to:
+70% to get a post-effect (burn/slow/poison/weaken, etc.)
70*5=350% more damage from spells - but that's not all!
70/7=10, so 9. 10x15=150% more damage from spells, so 350+150=500% more damage from spells!!! (considering the 5% per point, 15% every 7 points bosuses)
Certain spells duration/power improved by 3 (every 20 points improve it by 1, so 3x20)
And it can go even further
I'm uncertain if the pain skull/Snake ring/Moneylender's ring/Ring of Intellect can spawn twice. I've never seen 2 pain skulls in my games. But there's plenty of room for improving a mage. I think I'll do a fun test game to prove how far it may go sometime in the future.
EDIT: Actually the maximum basic value is 70 - the mage has a medal that adds up to 3 intellect.