Your math doesn't seem to add up. 6# of LME in 5 gallons will give you OG of 1.043. Together with the grains it should be ~1.048 and your ABV then would be ~4.7%. The difference in IBUs with this gravity, if you boil full volume vs. just boiling 2.5 gal, is about 10 IBUs. (29 vs. 19). By style...