I don't own a Fit yet, but can offer some answers to your questions.
The Fit has 109hp (IIRC). I don't think you could really call them "fast", but spunky maybe.
There are advertisers here who offer upgrades like superchargers. I think there is a thread about a Kraftwerk upgrade.
Pretty much any upgrade you do to gain more hp will result in lower mpg. Not so much because of the upgrade, but because the upgrade results in a heavier right foot.
Yes, there is a 2009 Fit which is a redesigned model.