that means nothing becauser after that your mostly just blowing hot air. Call GD and he will tell you the same. Why don't people run 20PSI on stockers all the time??? Umm...because with that small of an exhaust it isn't effective and blows mostly hot air. as I said....turbo efficiency comes into play
There are a number of people who have made 400 on stockers without spray/race gas. An intercooler sprayer will help do the trick. I'm not saying its wise to go lean/max it out to 400, but it has been done plenty of times.
