Well besides being a terrible idea to combine BB and SL, you would be going up to 4.5" of lift all in the suspension. So here is a list of required components besides the BB spacers and lift springs, SYE/CV, Adj CA U/L F/R, Adj TB F/R, Sway bar Links, Ext Brake Lines, Bumpstops and shocks. And they are going to have to be compatable for a 4"-5" lift since that is how much total lift you will have in the suspension. So the components that come in the 2.5" SL kit will pretty much be worthless. Not to mention the terrible Suspension Geometry and terrible ride that this setup would produce.
As Dextreme stated, do yourself a favor and just go with a BL or replace the BB with SL and BL at the same time.