Another question....I`ve bought a BTC worth of STEEM on Bittrex, sent it to Steemit, converted it to STEEM Power and now the USD amount shown is roughly 400 USD.
So how did I manage to loose roughly 250 USD in the process?

Haha... you haven't lost anything. The $figure on Steemit is an estimate (
it's based on an average price over 7 days I think - the White Paper goes into detail about this). Your Steem dollars would be based on whether you sell it for on the open market when you power down.
This. The value displayed (indeed all the dollar values) displayed on the site are using a historical average. Assuming the price remains at least at its current levels, the dollar values displayed will climb over the next several days.