What it does it makes it so that your transmission wont lock up till a higher RPM. So basically It allows your engine to rev to 2300 RPM before it engages the drivetrain so that you are in your powerband when you launch. it would be like letting the clutch out on a manual at 2300 RPM instead of immediately. Your car already has one but its at a lower RPM so when your drivetrain engages you dont have as much power. Its a great mod to have on an auto for drag racing.
A tq will NOT, *NOT* ****NOT**** change your gear ratio. This is primarily going to be used for racing purposes and a high stall tq is not recommended for street use.
A tq will allow you to rev the engine higher before the car starts to move. A 1200 stall tq will engage the transmission around 1200 rpms. Anything below that would be like neutral (right?). So the higher the stall, the more rpms you need to generate before the car starts to move. A 5k stall would be quite painful to drive around on the streets.
I'm still on the stock tq and haven't had any issues to mention. The higher stall tq would certainly help me at the track with launching, but would make the car less streetable. I don't know what a streetable high stall would be. 1500 maybe? 2000? They go up as high as you want, but the higher it goes, the harder the car launches.
Remember, you will be launching your car from whatever the stall speed is everytime you go from a stop.
Go try and launch your car in a parking lot. Left foot hard on the brakes (really hard!), right foot hard on the gas (traction control OFF!!!). You will notice that the car will overpower the brakes. With a higher stall, you could rev the engine much higher before much power is transferred to the tranny.
There are a few IS' with upgraded tq's running around. I bet you can find them if you run a search. You should ask them what stall they are running and which tq they have.