I did a few conversions recently, and yes, generally speaking, If your goal is to power up it makes sense to buy SBD and convert rather then to buy Steem. Here's one example :
100 Steem => 54 SBD =>convertion=>125 Steem
However right now I can only buy 52 SBD for 125 Steem, so in SBD terms it was a loss.