Actually, the amount of boost for a stock unopened factory engine can be very high. This is because the amount of boost isn't getting into the cylinders as good as an engine with ported heads, intake, larger valves, better intercooler...etc. There is a point of no return when the engine doesn't make more HP with more boost. This is the reason why you can run 35psi of boost and only get into the 10's with an unopened engine. You can run much lower boost on a modded...ported heads/intake...larger turbo...larger intercooler set-up and run faster. Just remember 500HP with 30psi of boost is the same as 500HP with 20psi...it's all in how much you want to spend for upgrades.