The 5 piece on Pelinal's states "Your Weapon Damage and Spell Damage both become the highest of the two values".
I have noticed that when I use this set and raise my Weapon Damage to say, 3500, which in turn raises my Spell Damage to 3500, Major Sorcery does not do anything to raise either value. The same thing occurs if you were to raise your Spell Damage to 3500, which in turn raises the Weapon Damage to 3500 due to the 5 piece effect of Pelinal's. In this case, Major Brutality doesn't change the values at all.
In the first situation in which my Spell Damage is made equivalent to my Weapon Damage I am fairly certain that the way the code is working here is that the game is treating my spell damage as if it isn't affected by Pelinal's at all, thus when I use Major Sorcery it's treating my Spell Damage as what ever value it was before being changed to the same value as my Weapon Damage.
The same thing occurs in the situation where Weapon Damage is made equivalent to Spell Damage. When using Major Brutality it is treating your character as if the Pelinal's set isn't actually changing the values on your stat sheet.
I am just wondering if this is a bug or working as intended, because it seems to be really affecting the potential for any kind of dual damage build or unique builds using the Pelinal's set.