You have to consider that you have a 320 ft run of cable that is losing some power. Also, our power supply is somthing like 90% efficient from 48V to 5V (which is very good) -- but it means that you lose 10% there (turned into heat). The RB14 has a linear power converter that loses about 20% going from 5V to 3.3V for the mpci. So, you may need more than 19.2W. We initially sold 24W power supplies and now we sell 30W power supplies as they run cooler and will be able to supply our RB300 also.
Another potential problem is power spikes in the long POE run. If the Ehternet is mounted next to the grouding rod or a power line going up a tower, then it could get problems from those lines. If it is in a building, then it shouldn't be a problem as there is so much metal criss crossing in modering building and that grounds everthing.
We have an expert on this that is doing some design work on our newer power supplies and we will try to get him to write a paper about this subject. Analog power design is extremely complex -- it is something artwork to get something perfect.