Run a dyno test on an engine with old plugs (and probably poorly tuned as a consequence).
Change the plugs with any plugs.
Tune the engine.
Run a dyno test.
What results do you expect?
Look at the controlled test at the links posted above.
Those plugs provided only a modest increase in power and mpg. The payback might have been OK (not substantial, but OK) if you drive your vehicle a lot.
