See this article that handles exactly your question. That source confirms your ranking, but reports about a factor of 10 better precision than the numbers you mention:
- NTP from WAN: +/- 5000 microseconds
- NTP FROM stratum-1 source on LAN: +/- 500 microseconds
- Trimble Resolution-SMT timing GPS module: +/- 15 microseconds
- u-blox NEO-6M GPS module: +/- 15 microseconds
As for the performance of GPS without PPS, the same site (different article), here, reports:
Initial tests with just the serial 4800 baud NMEA output and no PPS using Windows were disappointing. Whilst the device worked, the accuracy and jitter were no better than could be achieved with a broadband Internet connection and servers from the NTP pool.
And about that same solution, at the end of that article:
At the default setting, 4800 baud, each bit occupies 0.2ms, and the uncertainty with which the GPS sends out the data, and the operating system can report the time the data was received, result in a somewhat higher short term offset jitter, and no better daily drift than with multiple Internet sources. You may be able to do better using a high baud rate, and perhaps with a USB-connected GPS and a much higher high baud rate. It's possible that the jitter would be less with a UNIX system, but I've not tested that.