Hearth Build/Insulation Questions

[Bongiorno]

First build, have base built and hearth slab poured yesterday. Trying to figure out the best way to insulate the hearth without adding too much height. Originally wanted to use a 2" cal sil board with full fire bricks on top. We're having trouble finding the board locally in San Diego and are on a time crunch. Now the idea is to pour 3-3 1/2 inches of a perlite/Portland mix and use half bricks on top of that to keep the height down. Can anybody give us some insight on pro/cons of the cal sil board vs the perlite mix? Also any insight on if 3 1/2 inches of the perlite mix plus half fire bricks on top will provide adequate insulation to prevent heat bleed?