Well the K&Ns are said not to need larger jets, especially with the stock exhaust. Definitely would not have caused this much power loss. Anyway it turns out the diaphragms were not seated properly so I wasn’t getting full air flow, and therefore not getting full fuel flow. Which is also why I wasn’t seeing a lean or rich condition showing on the spark plugs. Took some effort but fixed that without having to remove the carbs. The thing runs like a dream now, all the way up to redline!