Interesting.
Being in the UK I used to regularly get 150 or a little under on occasions. So I regard 150 as being very good. Of course as noted that is when nice and steady with no packet loss.
I have played recently with it between 200 and 300 but the problem is at that level it starts to fluctuate on my connection. I have logged on at least once recently to see it showing 400+ at the arena menu and not even bothered trying.
Currently if I am thinking of playing I run ping plotter (freeware version) to get an idea of how things are. So I am inclined to accept that 300 can be OK, but only if stable and without packet loss.
For what it is worth I once had a spell of bad connections that showed to biggest delay on the Atlantic backbone. I emailed the company concerned with details including my pingplotter graphs not expecting anything (they were not my service provider) but was rewarded with a reply that it seemed the data was not going over in the right group (my words not theirs) or something to that effect. They did something, let me know and my pings were back to 150!
However the current traffic is now via another route and it does seem very variable. The downside of progress in terms of the now very heavy internet use.
The graph attached is from a few minutes ago around 22.50 GMT - one reason I no longer fly in the evening UK time!