A couple of thoughts on this.....
It doesn't matter if it's squeaky brake pads, a low performing engine
or poor gas mileage, the standard dealer cliche is "They are all like
that". (As per the shop customer service rep's advice, I've had to say
that a few times also)
Second, NEVER run spark plugs for 100K miles. (Although I did see a
Winstar go 250K miles on an original set once!) Even if they do last
that long, there is a greater chance that they will seize to the head
and you will have real problems.
Third, maybe your performance is better than when it was brand new, but
quite often the performance decrease is so gradual with spark plugs
that you usually do see a difference with new plugs that have replaced
old ones. The same way with shock absorbers. You usually see a big
difference when replaced.
I normally chance the plugs in my vehicles between 30K-40K, and of
course, use platinum plugs.

While the advertised service interval calls for replacement at 100,000
miles, many factors can influence that. Idling time, driving style and
conditions... the list goes on. There will be posts regarding "dealer
bullshit/factory hype" and so on. Fact of the matter is that "sh.t
happens"...... nothing will ever change that....

I will hazard a guess that it runs rough when cold, though....