The environment within an auto is very noisy, even with a well insulated and sound-deadened car. Depending on what compression level you are currently using, you may or may not notice a difference.
I have a 2007 Infiniti G35 with a Bose (yeah, I know!) sound system. Their are multiple audio sources I have used in this vehicle.
XM Radio. I think the transmission is around 128 kbps, maybe less. Sound isn't great, but it gives me radio in the mountains.
Hard drive storage system in car. Stores at 128 kbps. (there is a lesser alternative, but I pick the higher quality one as default). In comparison to CD input, bass is thin, midrange muddy, highs meh. Fidelity not much better than XM. May actually be worse in some cases as XM should be equalized for satellite transmission.
iPoad input to line in via audio out/headphone jack on iPod. Most of my iPod files are 320 kbps AAC, so reasonable quality. Using the headphone out from iPhone/iPod means SQ can be very dependent on volume level of iPod. Too low and not enough signal to get good audio. Too loud and get distortion from amp in iPod clipping. At best, it can sound better and fuller than internal Music Box hard drive system. But switching songs while driving and volume can move which makes it unreliable.
iPod input to line in from a line in power source hooked into dock connector of iPod/iPhone. True line-in signal. With 320 kbps AAC files sourced from iPod/iPhone, sound can be very full and powerful. Distortion low enough that I am comfortable blasting at very high volume with no worries of clipping or damaging speakers. Next to CD input, best sound available.
CD from 6 disk changer in trunk/single disk in dash. Best sound of bunch, but only marginally better than line in from iPod with 320 kbps files. Bass is somewhat fuller and more powerful, but otherwise, hard to tell the difference head to head. Given engine and road noise, once ears are acclimatized, I enjoy either source equally.
The other thing to keep in ming with iPod/iPhone is to watch the EQ setting on the device. Using R&B or Rock can boost the bass and be a risk to speakers at high levels. I believe the dock connector bypasses EQ, but not entirely sure.
In my experience though, 320 kbps gets you 95% to optimal auto SQ and saves a lot of room on the hard drive. JMHO, YMMV!