I do not recommend EPGY math (at least algebra up) as a supplement. Honors Algebra 1 was a real course and since it's expensive (paid by 3-month intervals) it behooves you to finish in a timely manner. It would be very good to replace math at school that is not challenging enough.

That said, we did it as a supplement, but were in a "race" to get it done before next payment due. I think it would have been more pleasant if DS didn't have school math AND afterschool math. But he learned a LOT.