I have read quite a bit about NTP and PTP time keeping protocols. While many sources claim that PTP is more precise than NTP for that type of granularity - the reason is not as clear. Why can't NTP be just as accurate if a stratum 0 reference clock is atomic from GPS and network delays are accounted for? A paper on the FSMLabs site, in fact, claims that if both protocols are implemented well (as in TimeKeepeer they say), they can both deliver similar precision - and that it really boils down to the ability to stamp time in hardware to better account for jitter in switches, routers, and operating systems.
Yes, it seems possible in principle that NTP can approach PTP. Although, if the requirement is simply sub-second accuracy, NTP can do that over nearly any network.
NTP can be very precise, the format allows for sub nano second resolution. Usually the NTP implementation doesn't get near that: distant clients over networks of varying quality, and without needing hardware drivers to get timestamps. PTP is more for short hops where sub-microsecond jitter at a high frequency justifies the cost of hardware in each device.
Accuracy Expectations The accuracy expectations of PTP synchronized clocks is in the order of 100 ns determined primarily by the resolution and stability of a dedicated clock oscillator and counter. The accuracy expectations of NTP vary widely, depending on the motherboard oscillator and counter. Typical results range from a few microseconds in PPS-assisted primary servers to tens of milliseconds in widely dispersed Internet networks.