I don't think that the Hz is viewable in runtime, since it's at kernel compile-time constant.
The only way change it is to to compile an new kernel, either with the kernel-packages in your distro or by a vanilla kernel from kernel.org.
Google is your best friend here, seems that ubuntu choose 250 as a middle way.

The problem with higher scheduling speed is that the scheduler takes more and more cpu, and in the end the scheduler itself will take more time than the processes it schedules if you have a to high system interrupt clock.

200 pieces a minute is 300ms/piece wich gives 15 PLC-cycles/piece at 20ms cycletime (30 if run at 10ms), shouldn't that be enough?

Also one maybe have to look into the possibility to design the machine smarter so the PLC doesn't have to control every step of the process if possible.
If a fotocell and relay does it more accurate, maybe that is a better soultion for that specific function?

Yes, that might be a solution. Another way is to use a separate PLC to handle very fast sequenses and perhaps control it over Profibus from Proview.

Anyway, the cycle time is not a problem in my application.

Regards
/Ohl