ok... this is simple... everyone over thinks this.
an L has a 2.9X" upper pulley the HD has one X.XX" that & ECM tuning are the main reasons for the 2lbs less boost. If you swap an L upper pulley or snout you will gain 2 lbs of boost. Period.
{edited:} Ryan explains this perfectly below... I was working off bad information & faulty logic...
the bigger issue is the discrepancy between one company's quoted "6# lower" versus another's which is not the same diameter. If they are different diameters they both can't be a 6# pulley.
A couple of the original guys started a table of Vendor "X" has a 6# pulley X.XX" in diam & Vendor "Y" has a 6# X.XY" in diameter.
All you can do is look at your boost gauge (you did put one in right?) & see what your truck makes before & after.
Rob W.