I wouldnt run anything over 10 - 12 psi at most. Your stock turbo will fail after that within a short period of time. They dont like having too much boost going through them.
The main reason as I know from dyno'd stock motors is that inlet temps run to high with the standard top mount intercooler. I have been told that this is the only reason you can't put much boost through them.
On the other hand their would be an efficient point of boost for the stock turbo. So their would be no point trying to push it above what it works best at for flow. Maybe because the turbo itself cannot spin as fast as the air speed this could cause trouble with the impeller and/or bearings? Is this what you mean old-skool? I did not realise this could hurt the turbo, just that it would run less efficiently.
If the turbo is in need of a rebuild then I guess putting over 12psi could hurt the turbo but I would be more worried about the top mount IC letting intake temps run to high which eventuates in Knock and detonation.