I burned a few at 8x and the rest at 4x. They all work fine on my unit, but I also used good quality media. If your burner isn't good quality or your media isn't good quality you will get bad discs if you burn them at high speeds. Bad quality burners will not catch their own errors and they'll think that the burn was successful. But when you pop that disc in your PS2, it won't read.
Who ever said that you should burn at the rated speed is an IDIOT! The speed written on the disc just tells you the MAX speed it can go. It's just like horsepower on cars. Most of the time they just write 255bhp, but the detailed specs say 255bhp@6500rpm. Does your car give you problems at slow speeds?
Just to prevent frustation and uneeded coasters, you should burn DVDs@4x, Data CDs@16x, Audio CDs@8x. The first two you can go with higher speeds, but Audio CDs don't have any error checking so they'll skip if you burn them way too fast, like at 48x or something.
US/NTSC V5 - Matrix Infinity v1.75. - Network Adapter - WesternDigital 80GB HDD - CodeBreaker v9.2