The alignment specs should be the same with the 1.5" drop, as long as you get it aligned after lowering the car, cause if you get it aligned first, then once it is lowered, the control arms will move and the camber measurement will be off. The castor measurement is the same regardless of suspension travel so that shouldn't be an issue. The only other thing that might present a problem is how the suspension moves when everything is 1.5" lower. At stock height, there is very little camber change as the suspension moves up and down through its arc. Once you get out of that normal range, the amount of camber change for the same amount of suspension travel could be much higher. This wouldn't have any effect as you are just cruising down a smooth road, since the suspension wouldn't be moving very much, but on a bumpy road, as the suspension bounces around, it could be enough to cause excessive wear on the edges of the tires. Unfortunately the only way to really find out whether this is going to be a problem or not is to do it and see what happens. I'd say set everything up, making sure the camber measurement is dead center, and check the tires after 3000 miles to see if they are wearing unevenly. If they aren't, leave them on for another 3000 and check again. If it takes more than 6000 miles for you to notice anything, then you'll be fine, just rotate your tires as needed. If you notice it before 6000, you'll need to make an adjustment to the camber to compensate. Which way you would go would depend on which side of the tire is wearing out faster. If the outside is wearing faster, you need more camber. If the inside is wearing faster, you need less camber. Also remember, when adjusting camber, you will also have to re-adjust the toe.
That don't sound good at all.