you want e PCIe cerd in a Northbridge slot for the fastest path to the CPU.
as far as ASIO, Cubase on a Mac is known to be slower, by a lot, than on Windows. I use it, VE Pro obviates most of it as I load very little into Cubase anymore, but do not expect tip top performance to keep up with the Joneses if they are running it on Windows and you're not. ASIO is not Core Audio and there appears to be a translation slowing it down.
Now, the fact of OSX is that there are no tweaks to the OS such as was the case when I used Windows, it was de rigeur and may be somewhat less so today but I do see it discussed. So the bottlenecks on the Mac Mini are quite concrete and more or less hardware-specific. [EDIT: you do not want USB devices with drivers, such as cameras and printers installed on an audio computer, especially if you're using USB for audio in any way]
you want al least 7200RPM hard drives that are not system drives, for the samples.
you must use fixed and not dynamic IP you set manually, or you will have terrific problems.
if you aren't needing 8 logical cores, the system will not distribute to them. that one is you trying to worry about something that works just fine.
kontakt does not need to have multicore support activated in VE Pro, so don't. it could mean some problems in some situations. also, certainly in 64-bit there is no reason for the memory server. if you have instantiated both Play and Kontakt, the latter with the memory server on, they won't tolerate each other.
I believe the best practice as per buffers is as low as you can go in the sequencer but 'x 2' in VE Pro.
if you are working in 32-bit, you may find that your memory ceiling in practice is not what you noticed on paper somewhere.