.. if this field is set to a 32-bit OS when it's actually a 64-bit OS, are there any performance hits associated with this mis-configuration?
thats undocumented.
guestOS = x sets an unknown number of vmx-file parameters that are not printed to the vmx-file.
The effects of this may vary depending on your host CPU, effective guest OS flavour , and what not ....
Setting this wrong may result in effects like Windows 2003 detects the wrong type of E1000 nic, a Linux guest may get an unusable mouse, an ESXi guest may not be able to run nested VMs .... just to name a few.
Most of the times it is best to get the closest match: for NetBSD guest use FreeBSD, for Windows 9 guest use Windows8 and so on.
If you are lucky jmattson will give you a good final answer