Here it is about Splitfire Triple Platinum Spark Plugs
"I have a 97 cadillac catera and after I installed these plugs I noticed an increase of power and fuel economy. Coupled with a K&N air filter I have gained at LEAST 12 horsepower mostly noticable at the low end and have gained 3 to 5 more mpg! it is a great product for real gains on obdII systems."
I've never had a filter or plugs add any significant hp. I could buy a couple hp and 1mpg or so, but 12hp and 3-5mpg is way high. I am getting a K&N filter for my Catera, but I don't expect any real gains in the power department.
If your engine was missing on half the cylinders and the airfilter was plugged to start with, you would get a pretty good gain in milage if you put in any kind of sparkplugs and threw away the air filter element. Believe it or not there are many cars on the road that are in exactly this condition. These are the customers this marketing is aimed at. If your car is hitting on all cylinders and the filter lets anything through you aren't going to notice any difference in anything but your bank balance.
I think you noticed no gains with the filterbox assembly off because you were sucking hot air from the engine compartment into the engine. If you use a K&N filter (not a cone) and keep the original air filter cover on, you're sucking in colder air....