Turbo head engines have turbo plugs which require no shim unlike standard plugs. An article years ago mentioned that the difference was about 3,000rpm at a certain part of the engines powerband. However, if out of tune it would perform as fast as a correctly tuned standard plug. I ran a turbo head with turbo plug on my RB V12 3 Port Rody and I did notice a slight difference. For the price to switch it isn't worth it. For me it was a test. Most .12 race engines use turbo plugs anyway.
Trying to live every day like its the last....