- primarily street driving, some "spirited". works decent in cold too (i.e. snowy weather when I'm in the mountains)
- no tracking
- want more bite than stock pads (though I'd figure all/most decent non-oem ones do)
- low noise
- preferably lower dust than stock, but I understand that dust:grip is usually a tradeoff, so fairly minor

It seems like a lot of folks like Pagid RS 4-2 (blue) and Porterfield R4S, but I'm wondering if those would be overkill for my situation?

As a side noob question, why are the pads so expensive for some of the Audis vs. every other car? (based on <a href="http://www.horsepowerfreaks.com/sale/Porterfield/Brakes/Brake_Pads/part/Front_-_R4S_High_Performance_Street">this</a> hit from google) I'm not sure how up to date the pricing is, but the same pads for the M5 are roughly half the price.

...first off, I'd skip the Porterfield R4S. My issue isn't that they aren't good, it's that they are inconsistent. I've had sets that worked great, and sets that sucked.

I back the Pagid 4-2 Blue for an aggressive street pad. I'd also add Ferodo DS2500 if they have a fitment for hte stock S4.

As to your sidenotes:
If you exclude the S4/A6-AR 2.7t and 4.2/RS6 Audi pads are pretty standard priced. The problem with the HP-2 caliper is that it's 4 pads per caliper, and there is two different backing plate designs. So for the manufacturer to make pads for the car, they need to cut 8 pads, and 4 are one design, 4 are another.

By comparison, most BMW's will have 4 pads with the same cut on all of them (I'm pretty sure at least).

Got any recommended local suppliers and/or installers? I'm not much of a gearhead, and this would be the first replacement out of warranty. Should the brake fluid be changed as well? Fluid recomendations?

...don't know any shops in SF directly.

As for where to buy, i'd hit up the S4 forum and see where people are getting pads for stock caliper.

Fluid, definitely worth a flush, I say once a year or more if you track heavy. Motul 5.1 is great, no problems, price is good. Run Motul RBF600 if you want, but the 5.1 will last longer if you don't need that last bit of temp range protection.

Streetwerke may be able to get the pads for you, probably worth a call and get a quote. I'd say 2 hours labor or less to swap front pads and flush all 4 corners. Expect to buy 2 500ml bottles to flush and top off the reservoir. Front pads are a 15minute/side thing, plus about 10 minutes to get the car in the air and pull the wheels off. With a pressure bleeder, figure about 15-30 minutes to bleed depending on if they are pushing fluid or just air (and thus need to disconnect and add fluid a couple times).

A quick shop would charge you 1 hour labor to do all this, start to finish assuming they don't see any issues while in there with the brake system. If they are asking for more then 2 hours labor, forget it.