This is a big change, but an important one. As written, if you lower your character's Per and Will, you'll see that IQ (just IQ by itself) costs 10 points/level. Compared to the price of Talents, Magery, and even skills, that's just too little. Now that mental skills cost more per level, it's unbalancing to make IQ cost less.

In addition, it makes themetic sense for Will and Perception to be divorced from IQ. Intelligence certainly doesn't affect alertness -- look at any animal to see that. And your strength of will isn't related to how smart you are, otherwise nerds would intimidate jocks, not the other way around.

This does slightly change the cost of building characters, so you'll want to mentally add about 10-15% to the starting character points suggestions in the books.

(Note that Affliction (Attribute Penalty, IQ) no longer reduces Per and Will. Instead, the Attribute Penalty enhancement may be bought for each at +5% per level. Similarly, Steal Will and Steal Per are +100% enhancements for Leech.)