E3 spark plug inconsistency

When those first came out, my dad's "go to" engine guy had ported a head for him to copy all the ports on. He commented on what spark plugs to use when shaping that area. Off the cuff he mentioned he and another customer were dyno tuning his engine and the customer brought a set of these plugs to try. He said they lost 20 hp loss across the board with no other changes. He never said what the engine build was but he did say to stay far away. Bosch had some similar plugs and I never heard anyone who had them like them either.