A smaller pulley will result in more boost, and ultimately more power. However you also have to account for tuning (whether or not you're on pump gas, more boost might not settle well with it, you could have detonation problems), and you'll need more fuel delivery also. I'm not sure that a T-Rex alone with a stock pump would be adequate, but that would come down to adjusting the fuel pressure and making sure the pump can keep up. More power will also provide you with more traction issues, but will make your car quicker and faster if you can hook it up. As for how well your car hooks, you can generally guage it based on how it's doing now and see if you think more power will hurt traction a lot or not.
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1991 LX Hatch 5.0L