First, let's talk about how I selected the pieces I ended up with.
Head Unit - I've always been partial to the tuners in Pioneer radios, but my primary motivation this time was their integration with Google. The NEX radios support App Radio mode, and even google voice commands when connected with a cable. Not sure if I'll really use this feature, but it steered me towards their products. The 4200 was the highest tier modem without going to units with built-in NAV (at a substantial premium). With a $100 rebate, I got the head unit for around $400.
Subwoofer - I picked this up from the classifieds some time back, and it sat on a shelf in the garage while I've been waiting to get the rest of the project going. I've built many subwoofer boxes for other cars, but the Cayman shape and storage options doesn't lend itself to plywood/mdf boxes. Rather than have something custom fiberglass formed in the car, the JL stealthbox was the obvious solution, and buying used means a reasonable price.
Amplifier - Probably the toughest decision. Going with a Class D amp is obviously prefereable (mandatory?) in such a small car. I know most on here go for a 6 channel and try to drive the factory subwoofer and/or 8" door speakers, either in parallel or with resistors. I couldn't see myself doing that with the Bose speakers, so once I resolved to get 3-way components I only needed 4 channels + 1 for the sub. I considered (agonized?) over many amplifier options, but consistently found the HD900/5 mentioned as the best Class D available - it is JL's flagship amplifier. I also considered the Alpine PDX-V9, as well as other JL options: XD700/5, or even an XD500/3 with the head unit driving the rear speakers. In the end I came across a non-working HD900/5 on Ebay, which I took a gamble on. Thought I might take it apart and find something obvious that I could fix. Nope. So I sent it in for service, JL replaced the internals, and for ~$400 total I ended up with a factory refurbished HD900/5 with a warranty.
Speakers - This was actually pretty easy. Since I already had a JL sub and amp, I felt it made sense to stick with the same brand for speakers. The 3-ways were easy, as the C5-653 is the only set JL makes. Rears were also pretty easy - but you have to go down to the C2 line to find a 4" speaker that will fit in the rear. It's just rear fill, so not a big deal. One note on 3-way speakers....the spacing between the door mounted midrange and dash mounted tweeter is not ideal, but it's what Porsche gave us. I'm not keen on cutting or surface mounting a black tweeter in the door panel, so I'm sticking with those locations. The key is making them fit (more in a future post).
Backup camera - Not strictly necessary, but if you've got the panels off and you're running subwoofer wire to the trunk, why not snake some extra cables and mount a camera? I picked the model I did because it has a tiny connector from the camera side of the cable to feed into the car. I figured if there was any chance of having to drill I wanted a tiny hole. I ended up pulling the wire through the taillight opening, and size was absolutely not a factor. So don't worry about that if you are picking out a camera - any size connector on the cable will be fine.
Microbypass (for parking brake) - some may question whether this is a wise choice. I have no intention of watching video or really even using he screen while driving. But when I have a passenger, I don't mind them being able to pair a phone, enter a nav destination, or use any number of menus/options that the unit disables when the parking brake is off. It's flat out easier to bypass it and let the thing work. And, it saves you the work of tapping additional wires under the center console.
Wiring - I've always liked KnuKonceptz for offering quality cables for a reasonable price. Ordered 4 gauge for the amp, although the runs are so short I could have gotten away with less. But 4 gauge is a better fit for connecting at the battery terminal and amp terminals, so I went with 4. KnuKonceptz Collosal Flex is aptly named, it's a 4 ga cable that you can coil around your wrist - crazy flexible cable. Also bought 12 gauge speaker wire for the sub, and 16 gauge for everything else.
Installation kit - went with an Ebay seller that seemed to have a decent looking kit, with Bose connectors. I was on the fence about pulling my own wire vs tapping into the Bose harness in the boot, so I wanted to get a kit that gave me that option. More on that later.
more coming...