Well, if gt5 should work ultimate online, you must fulfill these:
1) Decent internet connection, 8mbit at least
2) Decent router, since the type of router does affect much
3) Router and ps3 in DMZ, and static ip on ps3, or port forwarded
4) No other people abusing your network
5) Ps3 hooked up thru a wire
6) Clean cache files before you go online
8) The host must also fulfill these steps in order to host a clean room...
9??) Turning music of, that was a new one
Few people fulfill these....for example on my specific router i cant use DMZ, how ever i could open ports and give ps3 static ip, and that helped me alot. I also upgraded from 8mbit to *secret* speed, and that helped me as well. It also depends on the type of connection you have and how stable it is. I know for sure that manny people that are connected to fibre lan have big problems due to their lame ass provider. What more, well, if you have ip telephony and iptv, the provider some times install a "ping filter", and that also affects your latency.
Where to start, well visit
http://portforward.com/