Ideas and suggestions.

As an alternative: the artifacts, as usual, reduce special power, and the set additionally reduces it for both. This way, there is no one-sided imbalance, and the effect of the artifacts is preserved without the collection bonus. (It's not like King's Bounty at all.)
It will be useful for Warriors/with a superior army/with high combat stats. The numbers, of course, need to be tested.
For Mages with a huge special power collection, it will be irrelevant, but the artifacts themselves are good.