The efficiency is the same pre-boil or post-boil. So the boil has no effect AT ALL on efficiency because sugars are conserved.
I like to think of brewhouse efficiency in terms of:
brewhouse efficiency=conversion efficiency*lautering efficiency
because if we were all lucky enough to acheive 100% conversion AND
also lauter every single drop....we would have 100% brewhouse effiency. Of course this is virtually impossible but it proves out the calculation.