Updated to 2017.50.3 yesterday (it claimed failure, but the car appears to be running 50.3 and has the auto-sensing wipers (beta)). It seemed like it was doing better with Slacker with my limited use, but I've also learned that only time will tell if it is better or not.
That said, I've had a productive dialog with Slacker Support since I first contacted them December 27th. They shared today that Tesla acknowledged an apparent issue affecting the East Coast LTE service (no issues with 3G, and definitely not WiFi). The thought is that it's a network routing issue on their (Tesla) side. A subsequent update from Slacker Support stated that a configuration change was made around 4pm PST today, with a request to see if the buffering issues still exist.
I tested in my garage for about 30 minutes - WiFi connection disabled, one bar of LTE coverage. It did indeed seem to be working well for me. Buffering was good - always faster than the playback rate in the half hour I was bouncing around stations and songs.
So... Tesla Slacker fans - would be great to hear if anyone notices any similar improvements in buffering after 4pm PST today - and if you are on the East Coast or not. Firmware version might be helpful as well.
Keeping fingers crossed...