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A lot of it is based on reputation though. Gut himself was a RF in Cleveland and many thought Sizemore was the better defender at the time.
I agree that +10-15 runs is more conservative, but Ryan has had two straight rediculous defensive seasons. He looks great in the vid I've seen and he might have some more 20 run seasons left in him.
Bref had his 09 season at 3.4 WAR. While hes unlikely to hit that well again, he could pass the 3 WAR mark again with +20 run D and more playing time.