There isn't a fine line with stock turboed supras. The turbos are so puny that it would take a while of high-boosting to damage the engine. I've seen timing retard as low as 15 psi with the stock IC, even on a cold night.
Horsepower #s vary greatly depending on mods, gas quality, and air temp.
Given how hard detonation is to detect, and how long it takes to damage these engines, I'd imagine most people on this board have no idea if they are knocking or not. Many seem to think that you can always hear detonation, so if they can't hear a ping they aren't detonating
Larger turbos are more effecient in terms of intake charge, and leave less exhaust in the cylinder after the exhaust stroke. In other words, less of a tendancy to knock.
If your wondering how to select the amount of boost to run, here is what I did. I saw that I would reach full timing advance at 5700 rpm with minimum boost. At 18-19 psi I saw full timing advance only 300 rpms later. That seemed acceptable to me, so I left it at 18. Anything over 20 and it pulls timing more.