Race engines generally have a little more clearance than an engine that is built for long life, especially drag race engines.
Harley builds engines so tight these days, that they run hot and "feel tight" for 5,000 to 8,000 miles. An engine that is tight should not be run at high RPM even with no load, but that's just my opinion.
I know of no definitive study with regard to the relative merits of different break in methods. Fact is, it probably doesn't make much of a difference. Rings will seat no matter how the engine is run. Modern ring profiles and materials with precision bored cylinders mean they seal well right from the get go, and are probably fully seated within 50 miles. Pistons need a little more time to break in, IMO, and should never be stressed with high RPM, full throttle, or high temperatures until a 1,000 miles or so, but again, it's just my opinion and what feels intuitively right to me.