I recently bought a modded .21 engine for my race truggy from a respected modder and pipe builder, his break in was similar to the OS method of hit the ground running and not too rich so the temps can get to 200+F, keep it running as long as possible because cooling down just stresses the rod and bearings, continue until mechanical pinch loosens up.
PITA with a tight engine, and me being skeered to run it lean enough to achieve 200=F temps even with a sock on the head and not being "boggy rich" like I'm used to, also instructions were to not hit any high revs, even tho with the leaner settings for higher 200F temps it wanted to just rip.
Broke in for a gallon and the pinch loosened, still has killer pinch and runs like a top at 3 gallons.
Then there's my HPI K5.9, a "known bad engine" as stated on this and many other forums...
I did the factory 3 tanks idle on a stand.. in a garage while it was raining out, and continued the factory few tanks on the ground.
Ran the first gallon thru and didn't push the tune, and it still constantly did wheely induced rollovers, then leaned it out and hit third gear in the Savage XL on the second gallon.
3 years and 13 gallons later the engine finally took a crap because of the bearings, due to, IMO, the low oil content of the same O'Donnells Speed Blend fuel that took out the bearings in my 8.0 engine.
Byrons always kept the 5.9 nice and buttery looking on the inside, couldn't get it anymore.
IMHO break in is important, but not as much as diagnosis of air leaks (K5.9) and tuning skills over the life of the engine.