In a nicely QoS policed and managed LAN the de-jitter of 16 ms could be further reduced. I would say 8-12 ms. Do you have data about your network jitter? This cleary increases the chance for a playout-buffer underrun in the event of network jitter peaks.
The de-jitter works in finite units of packet, more than in ms. So with standard packet size, you can configure to hold 2,3 or 4 packets in the buffer. That corresponds to 8,12 or 16 ms.
Your target should be to hold the round trip delay under 50 ms. My experience is that 35-45 ms still is not perceived by humans. There is no hard treshold, except if you need to comply to a written quantified requirement.
You can decrease the packet size from the natural default to a smaller value, with the penalty of lower bandwidth efficiency. E.g. half the packet size will decrease the serialization delay by 2 ms. Not too much gain though.
In real life any VoIP link that allows telephony to the real world PSTN with analog subscribers asks for echo cancellation at the near end (before entering the IP cloud). In some installations where VoIP is reduced to low latency high-speed ethernet links it is possible to keep the delay down and be able to avoide echo cancellers.
