Good job on making the jump to manual! One thing though... where did you get the notion that an e36 wants 91? Again, being ignorant (), I only transitioned to 91 from 89 after it was sharked...
I appreciate the hard choice you have though... the GT-S is a bottle rocket when you hit the threshold. HOWEVER! If you run it at 87, it's not going to work (in my humble opinion). The 2000 GT-S runs something like 11.5 compression and in my opinion, would knock/retard likewith 87. I'm quite certain that it commands premium gas.
EDIT: some quick searches reveal:
http://www.theautochannel.com/vehicl.../russ0019.html
Fuel Requirement unleaded premium, 92 octane
http://www99.shopping.com/xPR-Toyota...RD-31945297540
The GT is not only several k less than the GT-S, it will also get you better fuel mileage, AND it does not require 92 octane gas like the GT-S.