Your situation parallels one I witnessed first hand today. Cab was down and made several pulls on a chassis dyno at my friend`s shop. We were trying to get a plug reading on e85 and installed one new spark plug in #7 that was one step colder. The car lost 100whp on a A B B A test. We put the other plug back in and the last "A" pull was right back to normal. 100whp due to one spark plug. That really blew my mind. It is amazing what you can pick up or lose with minute changes. I am impressed that you picked up that much mph with a timing change.