Yeah that voids the warranty... while getting a normal phone and updating the firmware doesnt.While branding of firmware on phones is rather annoying I would stay away from eMpTy eeN as a network.
Just use Nemesis Software Suite and change the product code of your 5800 to one of the european ones.
If you dont like the network get the phone toss the sim and use your VC sim. Theres nothing wrong with MTN's network btw