Well considering the spring with your kit is 14" 600 lbs, and that gives a ~1" lift, you may get close to 2.5" total with the 15" 650 lb spring (1" longer and a higher spring rate). I would think at least 2".
BUT you could run into serious issues with a spring that long if the strut is not designed for it. You might be near the full extension of the Bilstein suspension and then would top it out during full extension (tire droop), or the coil spring would bend and bind during full compression. Think about a little spring that you compress down between your fingers, and at a certain point it wants to bend one way or the other.
I think the Bilstein strut could handle that extension, because it's based off the 6112 which can provide a 2.5" lift, but that uses a 14" 600 lb coil spring, it just uses multiple higher spring perch settings which the Pro suspension does not have. So I personally would not recommend going longer than 14" due to the possibility of the coil bend and bind under compression. I don't know the guts or details of the FJ Bilstein suspension so please don't take this as fact. This is just my limited knowledge of how suspension systems work (some but not a ton haha).
Dobinsons is currently in the testing phase, I believe, of a coil that I think would be around 15", but probably only around 550 lb. So that would only get you another inch to be around 2" total above stock stock. But in speaking with Mike, he has concerns about coil bend and bind under full compression with a spring that long on the Pro suspension.
Bottom line...if you want 2.5" of lift up front, install the FJ Pro suspension, and then use a 1.5" lift spacer. Until someone tests a longer coil and deems it safe to use on that suspension, which may or may not happen. Yes TRD Pro 4Runner owners, you/we might not get a coil spring to provide an additional 2" of lift over the 1" you/we already have.
