I also have to note that the monitor gets power from the PSU and not directly from the wall socket. It only has the standard PSU monitor AC connector.

i also use 220V Devices on 230V outlets and did not encounter any problem yet.
Normally it should be fine with ~12% increased voltage.

Also had a Hairdryer who did not like around 245 Volts AC. The heating coil burned.

It's almost impossible to say without knowing how a device creates its working voltages. If it uses switching-type voltage regulation, it will most likely work fine on 230V. If it uses linear-type voltage regulation, it depends on the heatsinks. If they can't dissipate the additional heat, they burn and short, which may cause serious damage. If the voltage regulation is solely transformer-based (unlikely, but still possible), using any higher input voltage is a bad idea.

ps: using 220V devices in Germany is no longer safe since 2009. (see Wikipedia for details)

Maybe check old magazines on google books to see if your monitor was sold in the UK or Australia - both have 240V. If you want to be extra cautious 240-220V stepdown transformers are available.

I second the stepdown option. No need to put extra load on voltage regulators if it's not necessary, especially with uncommon legacy hardware. I use a Genesis 1 power supply with my Japanese Neo Geo AES, as its included AC adapter would convert my home 120V AC to 12V DC, and the system takes 10V (from Japan's 100V AC). 20% overvolting for no reason? No thanks. The 120V AC to 10V DC Sega MK1601 transformer is fine.

Most of power circuitry that i have seen using linear regulation also uses transformers, they don't simply pass 120VAC on a full bridge rectifier, filter caps and then a linear regulator, they have a transformer and optionally a regulator. if you have a 120-12v transformer and then full bridge rectifier and filter caps and a 7809 for example the aditional voltage on the input goes up 10v then it will rise 1v on the input of the 7809 you can apply the dissipation formula (Vin-Vout)*current to see how much the heat dissipation will increase.

I decided to get the step down transformer just to be on the safe side. Some of my hardware are too hard to find to risk burning them out.
Thanks for the help!