The space saver weighs loads. I had mine out for 6 months and only put it back in for a recent road trip. Does it even fit over the calipers on the front of an STI? Get yourself a can of that expanding tyre filler stuff.
Moving swiftly on - your wheels. Swapping to a nice set of forged jap numbers will save a good few kilos per corner, and as we all know 1kg saved off the rolling mass is like 10 everywhere else.
De-wing it too, or replace it with a CF one. My CF wing weighs in about 1/2 of what the standard WRX ironing board did.