Most all aftermarket heads don't have the valve seats machined perfectly concentric with the valve guides. There can be a good amount of runout which causes sealing problems which costs power. The seats need to be measured and machined if needed so they aren't more than a few thousands out.
The reason bare heads and assembled aftermarket heads are only about $200 difference in price is because cheap low quality valves and hardware are used to keep it inexpensive. Quality valves alone can cost more than $200 alone, not counting springs, retainers, keepers and seals.
The reason bare heads and assembled aftermarket heads are only about $200 difference in price is because cheap low quality valves and hardware are used to keep it inexpensive. Quality valves alone can cost more than $200 alone, not counting springs, retainers, keepers and seals.