Hi Gelbster,
if you've found a INA part number on your OEM Porsche hydraulic lifters and the lifters are exact the same, i didn't see a problem.
If not. i think it is very risky to use them, because the valves are lenghtend over time. So it is very important to compensate this lengthening. Else the valve won't close perfect. This will cause a less in horsepower on the one hand, but much more important it will prevent that the valve will be cooled. And this will cause damaged valves, valve seats and valve shaft seals. And this will cause much much higher costs to repair.
Here is a video that shows the function of hydraulic lifters:
http://youtu.be/lorANZ1Tptw
If the oil reservoir and diameter of the oil pathways in the hydraulic lifter are not exactly the same, the lifter can not work as designated.
Regards from germany
Markus