In order...
Sundance Kid: Carl's flashes your ECM - no piggyback controller needed.
Sledhead79: I am using the HPS can on my wife's 660. Chris Kasel over at Carl's Cycle did not recommend the SLP pipe with their kit, although it is offered on the website. In testing they didn't find a significant improvement with it, but may offer a single pipe in the future if happy with results.
To all: If you haven't ridden one, you won't "get it". Really. Carl's has been building 660's since the earth cooled, and they work, period. You won't get an inflated dyno # or horsepower claims from them, but you will smile from ear to ear if you ride one. The one I built for my wife runs alongside (not ahead or behind) my backup 2013 M800 S/P Ltd (not ES) with a 162 x 2.6 track, clutching, and can. This same M800 will dust a stock Pro RMK 800 with a can, hauling my 225 lb carcass around. This alone tells me that the package works well. It also runs 62g belly buster weights @ 8000' and holds RPM (The 800 uses 10-62's, not sure if it will pull the belly busters, as they hit hard). It is a fun sled to ride due to it's low crank inertia and quick revving power, and the 600 motor is reliable as a hammer thus far.
This is just a different take on riding for about the same cost as a stock 800, with a much higher fun factor IMHO. Those interested in knowing more are welcome to PM me.