Justin, I'm using a 160GB iPod classic with approximately 19k songs on it. It doesn't take too long to build the library and I would say scrolling is very smooth without much noticeable lag. It really starts playing very quickly. I'll unplug it and plug it back in to see exactly how long it takes, but it didn't It's not COMPLETELY smooth, but very smooth and usable. I actually find trying to scroll large distances using the touch screen somewhat frustrating and have been using the right knob to scroll and select more recently. I don't have a multi-function steering wheel and find using the MFI with the steering wheel stalk to navigate large lists also a bit frustrating. These are pretty minor complaints, however. I'm actually really happy with how my iPod is working.
Charlotte, Siri is working fine when I have my phone plugged in for CarPlay. But, when I hop in the car for a quick trip many times I just want to leave my iPhone in my pocket. It would be nice if I could still activate Siri to make phone calls or control bluetooth streaming music in this case.
Generally speaking, this software update is a big win. :-)