Well if the shackle is advertised as providing 1.5" lift or is advertised as just being 1.5" longer, those two descriptions are entirely different.
If advertised as being 1.5" longer than a stock shackle that means you will get half of that in lift height (granted your shackle angle is perfectly at a 45* angle)
If advertised as 1.5" lift, then you will probably get that or very close to that in lift.