its not entirely ping. I have played games with 3 or 4 or 5 all above 400 ping and it's a little slower but not laggy or spikes. The game has an internal lag anyway, so up to 300 makes no difference at all. In peer to peer the BIGGEST thing that will lag games is everyone's speed. Don't confuse that with ping. Ping is relative to distance between servers and the time taken for a signal to send then return. What's important is how much data can be sent at once. It's like traffic on a highway - one lane = one car at a time. Lots of cars trying to merge into one lane will have to stop and wait one at a time.
Except for dial up and mobile network, most download speeds are fast enough. The biggest killer in this game is upload speed. How many signals your pc can send to the other 5 players in the game at once. Ping is important because over 450ms you are delaying the game by some 0.15 or more seconds every time the peer2peer games sync.
THEN if you have a router and or a firewall, and you don't allow exceptions or port forwarding these will block and even slow traffic, causing more problems. I've played games with 45 ping 1v1, and because of upload speed of the other player the lag was worse than 2v2 with 3 pings over 500.