If I bought a nice new car with 100,000 mile specified plugs in it, I'd probably run 'em for 100K & only pull 'em if something seemed wrong- that includes gas mileage. Why?
*Way* back when the Neon was still under warranty, it spit up a storm when started one muggy humid morning, & check engine light came on & would not go off. I took it back to the dealership, they checked it on the computer, & the mechanic's worksheet said he pulled the plugs, checked gap, re-installed. That was at ~30,000 miles.
The owners manual specs plug replacement at around 35-40 K, I believe. I finally pulled & replaced those original, pulled-once plugs- at 89,988 miles! They still started & drove fine, only missed at nearly 6000 rpm or above.
What were those near-miracle plugs? Plain old Champion copper-core factory installed cheapies: RC9YC.
So if they say a new set of iridiums will go for 100k, I'm willing to give it a try.