well of course its going to run rich. The stock eec program is designed to run at a WOT a/f ratio of 10.98:1.....so basically 11:1. This is a little rich on an SC car, but its a safe setting for a factory car. It all equals out because once you add more air via the blower, the car is running high loads, but the a/f ratio is correct.
Now on the other hand, if you remove the belt, very very very little air is getting into the motor. The computer is still going to dump in the extra fuel, and still shoot for an 11:1 ratio. because thats what its comanded to do. N/A cars run about 13:1, and you're running 11:1. So there isn't much you can do about it.
Honestly...i think removing the sc belt is a waste of time, I mean, come on, how much more can 93 be over the 87? Here its $2.08 for 93 and $1.89 for 87, thats only a few cents and adds up to about a dollor when you fill it up. Heck you spend over a buck on a bottle of soda.
As for removing the octane plug, that wont save you any gas mileage. All it does is retard the ignition about 4 deg under wot, so there is no detonation with a lower octane gas.
IMO, i think you'll see the same gas mileage and its not worth all the added trouble. Just put the belt back on, and drive the car normally. It wont suck down gas if you drive it like a Caddy. Just my opinion though.....