A little lean - clean burn, hot egt, good power
Very lean - undesirable explosions in the engine

The biggest difference between the R's and the Turbos is that the R's have increased fuel delivery, and more aggresive fuel mapping, but only to the tune of about an extra 14hp and ft. lbs. Because of the increased fuel delivery, the mapping can be more aggresive in the critical 3-4K rpm range.

Haven't you ever noticed that the 850 feels a little sluggish in that range? It's because the ECU is dialing back timing and boost to prevent detonation. With the increased fuel in the R, this does not need to happen. Plus, because of the cylinder cooling of increased fuel delivery, the R does away with the overboost failsafe. The stock Turbo's only hold max boost for about 7 seconds, while the R has unlimited full boost until a higher overboost limit is detected.