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I'm not that worried about Felix's ability to get people out even as his velocity declines. For example, in the last four innings of his 2012 perfect game he threw 78 percent off-speed and obviously nobody could touch him. It's much more a question of control. His stuff moves so much that keeping it in or near the zone is the biggest issue.
Speaking of the perfect game, you can look at GIF's of every pitch in the game at off-speed.com.