Since the current situation is that Silverfast 8 has issues with serious contrast shifts when scanning in higher resolutions in Mac, I resorted into the least comfortable solution; installing the software in Windows 7. Luckily I have Windows 7 installed on bootcamp partition.
According to Silverfast support, apparently the contrast does shift in Windows 7 version of the software as well but it is less dramatic.
They told me that the new version of the software (which should have the issue fixed) will be released in first days of 2013.
Now, looking forward how scanning in Windows 7 goes.