If it's unbranded there are no problems usually.Why are SI numbers important? I flashed
9.2.A.0.295 - Unbranded *James Bond Skyfall Edition* - Central Europe (1268-3194)
to my USA Xperia TL this weekend without paying any attention to SI number, and it seems to work flawlessly. Do I need to match SI numbers in the future?
Thanks
Custom number is just important if you need to send the device to assistance service or if you decide to sell. In these cases it's better that the custom number match to the original one.
Anyway sometimes it could happen that something (like wifi frequencies for example) doesn't work properly if the country from which the rom comes has different laws.