I agree with what Drak said. I would never use wood that I thought would result in a guitar that's heavier or lighter than what I'm going for in the final product.
This becomes even more important when the customer has very specific desires. For example I've built a lot of guitars with varying weights from a 4lb mahogany single cut to a wenge double cut that was a bit over 10lb and they all sounded amazing. But the lighter guitars were *much* easier to play for long periods of time without fatigue. I warned the customer that the solid wenge guitar he was asking for would weigh 10-15lbs but he said that's exactly what he wanted and he was thrilled with the end result.
I didn't notice any less sustain in the 4lb guitar from the 10lb one. If anything, the lighter guitar was slightly louder and more resonant unplugged, but the sustain was so similar plugged in that you could contribute it to the other design differences and not the weight.
You should be more worried about building the guitar you want to, taking weight into consideration more for comfort and less for tone.
.02