I'm looking at a 2013 IS350C I'm curious what changes were made I only found upgrades to the infotainment system ?

Any reason to go with a 2013 over a older model.

TIA

You're correct. However, the addition of HD Radio, App support, and refreshed interface goes a long way to modernizing the car.

Was this update also applied to the 2013 IS 250/350's?

Yes it was. Minor changes but if you live in an area with HD radio support, much appreciated. Also seems to provide traffic data now too. The only other change for 2013 that I can remember is the addition of the new F-Sport packages.

So this part kind of confuses me because there was a 2 week Sirius XM promotion that ended on 5/31, and that was when my traffic started working, but now that that promotion is over, my traffic is for some reason still working despite the XM radio being deactivated...

Does anyone happen to know the Part number / ID number of the 2013 Mark Levinson Radio for the USA?

If the sat radio antenna does not have a signal when the deactivation packet is sent your service might remain on until the expiration date of the tier and/or until they re-send and it gets the packet. It's a nice state to be in sometimes.