Unless my math is faulty ... because of the shock's angle, a 1/2" drop in shock length will produce more than a 1/2" drop in ride height. Without knowing the exact angle it's impossible to calculate exactly how much the drop would be. As a side note, I put the Burly lowering kit on my Street Bob (around $300 for front and rear), and I love the height and look. But, the ride is definitely more harsh.

 

If you do not want to slam it and make it too stiff, you can look at the Progressive 430 or 444 in an 11.5"
This will still give a nice ride with good ground clearance. Also look into having the seat shaved, this will help dramatically.

 

Progressive's are a nice product, I went with 11" Full Cover on my 97. At the lowest position the bike was actually too low. Crank them up a couple of notches, even two up I felt the ride was an improvement over stock.