Hi John,
I don't have any experience with BSD, but I have heard only good things about it. It seem like the speed of a postgres DB would depend on a number of things like the skill of programmer in setting up the indexes, the skill of the programmer in constructing the SQL, and the capabilities of the the hardware. Getting enough hardware horsepower is not a problem since Linux will run on anything from a simple 386 computer to an IBM 390 mainframe.
There could be other advantages to BSD. I think Microsoft purchase a large block of stock in BSD within the last two year. Microsoft annouced that it would port it .net framework and C# to BSD to build the inoperability it needs to work on the Linux plateform. I have not heard anything lately about this, so I don't know where it stands.