OK, Back on topic.
If you want to run your engine like an old lady, run it in like an old lady.
If you want a lively engine, learn the engine how to work. Give it full throttle, but for very short bursts. Motocross is excellent for breaking in. So ia working like with a chainsaw. These short power burst are never long enough to cause lots of heat built up, because the surfaces all still have much friction, and contact area to pass the heat on to the outer surfaces is still small. (on micriscopic scale)
After a while, the engine limbers up and all parts become real smooth so the coeeficient of friction lowers, and thus the heat generation. This is quite noticeable, and about the time to reduce the oil content to normal running ratio. For running in the content should be high, and forget all the crap of different oil for the first gallons. Use the best oil you can get from the get-go. You do not want to wear an engine, you want smooth surfaces.
The second issue is heat cycling. During processing of all the engine parts, stresses have been set up. These stresses can be relieved by aging, but higher temperatures work faster. Because they work so fast, you must be carfull not to relieve it all at once. So stop the engine often to cool down, and then give it the whip again. Like in interval training.

Piston fit should still be tight, because the good match between piston and wall is what it needs to transfer heat to the cylinfer wall. Loose fitting pistons tend to sneak up on you and suddenly seize. Tight pistons start lumbering first with ample warning of being maltreated before they lock, often to free themselves after cooling with very litle, or no damage done.