Yes, I would build around Sandy Bridge now rather than wait for Ivy Bridge.
As far as I researched, given the same clockspeed, an Ivy Bridge CPU is going to have less than 10% performance increase over Sandy Bridge. Some estimates are as low as 6% performance increase. Therefore, Ivy Bridge CPU will need to rely on increased clockspeed to provide a good performance increase.
The small increase in efficiency is probably due to mature state of the architecture which basically means if you want better performance with the same clockspeed, then you need to make significant changes to the architecture which would not make sense at this point in time since Haswell is Intel's next generation CPU architecture for 2013.