I wasn't entirely convinced until I put the current ones on. For a while, the bike was running ThickSlick Deluxe tires, and on flat pavement it was consistently about 5% slower than my road bikes; on a route I frequently involving a pair of 20-mile segments with a cafe stop in the middle, I was clocking 19-20mph versus 20-21mph. The current tires made the discrepancy vanish.

I don't actually race on the road, but I do ride in a group that regularly leaves me nicely conked out for the rest of the saturday. The only performance issues I've noticed have been related to total weight, of which the tire width is a small factor, if comparing the overall build to my Emonda ALR (the Stumpjumper totals about 6kg heavier). On the flats, I'm basically able to ride the same and with the same people, only noticing that the power burst to hop onto the back after a pull needs to be slightly bigger. Rolling hills, obviously I've had to shorten my pulls to keep up. I don't see why a more weight-conscious wide-tire build would be at a very significant disadvantage, as long as the wide tires in question were a high-performance model.

What about it?

In theory it's probably increasing effective body-to-body paceline spacing compared to the Emonda, although I imagine that the effect of a couple inches isn't huge.

People sometimes bring up long wheelbase up as a cornering issue, but at least in the context of road riding this doesn't really make any sense; turn radii are huge compared with the length of a bike, and even long bikes need very little steering column rotation. It simply isn't a limiting factor.
The actual big limiting factor in road cornering is contact patch stability/grip. And, as demonstrated vividly by motorcyclists using armored pants so that they won't obliterate their knees when they scrape the ground, wide tires help quite a lot with this. The Stumpjumper is actually significantly more confident than my skinny-tired bikes in hard cornering.

I'll have to figure that out when I decide to try cyclocross.

Likely would have given it a shot this season, but I've been injured. First hurt my hip in an overuse injury in September, and a week ago, just as I was starting to get my cycling fitness back, I fell on a staircase and tweaked my back. So... maybe next year, if I can get back up to speed.

If you're referring to Michelin Pilot Sport, I'm doubtful. Those are beefy urban tires that don't even weigh less than my 53s. They might win out in edge cases, like if you mount them to ultra-wide aero rims and run them in a 30mph+ time trial, or if you're riding through semi-rigid day-old snow slush which cause wide tires to waste a lot of energy throwing ice around. But they're probably far behind in terms of rolling resistance.