Before I started writing this, I assumed there would be little if any correlation between spring training reports (of weight loss), and increased level of performance. Most players chronicled here, actually did worse.
(I was going to make this a FanShot, but it wouldn't let me both quote and link)
After the recent Hafner-lite bar sighting, I thought this article might be of interest. 2008 Indians on the list include Garko, Victor and The Close, all having lost weight and having under performed in 2008.
Hafner's weight loss is due to his recovery from surgery. Whether he will keep it off longer term is unknown. Personally, I think his performance is going to be 98% determined by his health. If he comes back healthy, even if much lighter than his listed 240, he will be a big contributor.
But for those of you thinking that a leaner Hafner might be ideal, this is some evidence to the contrary.