I wouldn't imagine that removing the spare tire would drastically change the handling of a 3100 lb car. However, it probably wouldn't hurt.

I'm not new to autocrossing but I've only run my WRX twice. The first time was with a full tank of gas and equipped with all the "stuff," including a trailer hitch.

Today, I ran 3/8 tank and removed the following:

Rollout rear cover (wagon) ~ 3.5 lbs.
Rear headrests (2) ~ 3 lbs.
Tool Kit ~ 1.5 lbs.
Spare tire ~ 26.5 lbs
Spare tire cover/tray ~ 1 lb
Jack wrench ~ 0.75 lbs
Jack ~ 4.25 lbs
Rear floor mats (2) ~ 2 lbs
Owners manual ~1.75 lbs
Ft floor mats (2) ~ 3.5 lbs

Sub-total ~ 47.75 lbs.
Minus ~8 gallons fuel ~ 48 lbs.

Grand total ~95.75 lbs


Did it understeer more? It didn't feel like it. It actually felt more tossable, but I wouldn't classify the change as drastic by any means. My car tends to understeer when you let off the throttle, unlike rwd cars, so if you enter a turn the wrong way, plow city. If you do all your braking before the turn and power your way through, it feels quite neutral. The two EVOs that ran today also appeared to behave in this manner.

Just my .02

 

I think you may be right. I felt the same problem. The rear end is very loose specially at 100 plus. I added a cusco adjustable rear spoiler and it really helps. Also my car is now completely gutted and the GT spoiler really helps.