Getting a wideband is a smart investment. I have an innovate lc-1 with their xd-1 gauge. It works well for me and the Streetuner em. Here are my two cents, mostly going off of the innovate instructions.

You have to install it post turbo. The backpressure in the manifold/uppipe throws the readings off considerably, with no way to compensate. There are really expensive lab grade wb/gas analyzers that can, and of course the factory sensor, but all average consumer wb's can't go pre-turbo.

If you have a divorced wg setup, you should install the sensor after the merge. If you have a bellmouth, it basically goes like this; The closer it is to the turbo, the faster the readings, but easier the chance of over heating the sensor. They really don't like to exceed 700f, but you can build a heatsink if you need to mount it that close. Obviously if you locate it further down stream, you lose reaction time, but you gain sensor longevity.

On a side note, I have a divorced wg, and the wb sensor is in close proximity to the catlyist monitor (2nd o2 sensor). There is definately a time delay between the readings I get from the factory psuedo-wideband and the innovate, but it all depends on gas flow. At idle, you could count a second or two between the sensors, but at wot, it's really damn close to what is actually moving through the engine at that time.

Sensor orientation is really the most important aspect of the entire install.

Condensation forms all the time in cold exhaust systems, and that water pools towards the bottom of the exhaust pipe (6 o'clock cross-sectionally speaking...). Mounting the sensor at 6 o'clock is asking for issues because it's not designed to get wet (and remember, it's a heated element, so thats the real conflict here). Innovate recomended orienting between 9 and 3 o'clock, but pushed that 10 - 2 would be better. Way less chance of damage due to inevitable condensation.

I know that you already bought one, and I don't mean to burst your bubble with my next bit of info, because the aem is a great tool, but it only reads to about 11 to 1 afr. The factory mapping in a wrx goes as rich 9.5, and has a bunch of cells that are in the 10's. Most people wind up making the best safe power in the low 11's after a tune, so in theory it should work out, but if you know the joke I'm making a crazy reference to, "In theory we should all be millionares...!"

But hell yeah, the aem is great and a wb will give you great insight as to what's really going on. Best of luck. (edit: pull the dp for best results! )

later, x