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If you screw one of these Iridium plugs into a TT head, they do reach all the way into the cyl.
The extended length of the TT plug extends into the cyl past the opening, by the amount of the area that extends past the threads of the plug.
No one makes a plug colder than a 6 heat range that has the extended reach.
I use to strongly recommend only the stock length plugs due to the obvious length difference.
We talked to NGK at length about this a number of years ago, on my own car the stock plugs were getting cooked with the #6 platinum's. NGK told us the Iridium plugs were far superior and suggested we should test the closest length available in a colder range. They said the threaded area is the same and they guaranteed they would fit all the way into the head.
We checked them in a head off the car and we were surprised to see that they do fit all the way (But just barely) to the end of the TT spark plug boss.
We then dyno tested them against the platinum's.
The Iridium plugs make slightly more power under higher boost levels, and basically the same power at low boost levels. I also stopped cooking the plugs on my car at higher boost levels.
We have been running them in our own cars and selling them with zero issues for about 7 years now.
All my hard passes running 9's, I was running the iridium plugs. All of our local customers run the iridium's, and almost every car we have tuned in the past few years has had them installed as well.
The stock 6 heat range is good for 9 PSI boost that the car came with.
Running 14 to 18 PSI all the time requires a colder plug, we recommend heat range 7 for this level, 19 to 24PSI requires heat range 8, running over 25 should be 9 heat range.Each plug number higher has the ability to pull 200 degree's from the cyl's, running too hot of plug hurts power, makes the engine more detonation sensitive, cooks the rings and plugs. The plug length is not perfect, but when you turn up the boost, it is a much better solution than keeping the hot heat range.
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