There's no changelog. However, it has been reported that versions 4.0.23+ connect substantially quicker. I was running 4.0.19 and had given up on android auto wireless, as it can sometimes be unreliable. Additionally, after driving for a couple hours, my phone gets pretty hot and performance drops. So, I was connecting the cable every time, taking about ~10 s since startup.
After updating to 4.0.32, I didn't notice any speedup with USB whatsoever. To be honest, it was sufficiently fast before, and I just wanted to fix a bug that drives me mad and has been there for ages: after using Google Assistant, the volume drops so you need to use Google Assistant again and cancel to regain full volume - this bug hasn't been fixed in 4.0.32. Wireless connection does seem faster, about ~15 s since startup. However, I believe I'll keep using it wired, as it's just more reliable (it connects every single time), doesn't drain my battery and my phone runs cooler, making it perform much better.
So, if you use wireless android auto I'd say it's totally worth updating, you will shave 5 to 15 seconds at startup and maybe it'll be more reliable. Otherwise, save your time, 4.0.17 is perfectly fine.