you'll def throw a CEL, but it will not hurt the performance of your car. Rear o2 is just used for catalyst monitoring. You don't need to build an o2 sim. There's a 5 dollar fix here and the only tools you need is a wrench, vise(optional), and a drill w/ 1/2in drill bit.

HELP brand spark plug antifoulers, 18mm, part number 42002. ($2.99 at most auto parts stores. I got mine at autozone.) You'll need one pack, which contains 2 antifoulers.

Take your drill with the 1/2inch bit and one of your antifoulers. Drill straight through this antifouler, widening the hole at the end and turning the antifouler into a straight little metal tube. This will take a while, and get quite hot in the process.

Screw your modified antifouler into the un-modified antifouler. Notice that you've created a little "chamber" for your O2 sensor, with a little hole at the end. This little hole will be exposed to the exhaust stream, and limit the amount of gas that the sensor sees. This effectively shields the sensor from the direct gas flow.

Screw the sensor into the modified antifouler. Screw the whole assembly into the O2 sensor bung. It will look like this.

O2sensor > Modded Fouler > Unmodded fouler > O2 sensor hole.

I've had this on 3 vehicles, 1 being my Evo, no CEL's ever.

Here's a write up on Evom
$5 o2 cheater/non fouler how-to - evolutionm.net