C'mon Victorian guys where are you???
Not from Vic, so no idea on MX-5 dedicated or specialist Mechanics there.
BTW Facebook is where everyone asks questions now sadly. As you can probably tell the forum is a bit unused. Don't expect to much of an educated answer to any question you ask though through Facebook though.
As the others said a SP is just a bog stock NB8B MX-5 with a Garett 2560 or 2860 Turbo. They have plumbing to suit including intercooler, pipe work and exhaust. I believe the ECU is just the NB8B ECU. Chiptorque in QLD, cracked them for Mazda Australia from memory. Hence why they offer tuning factory ECU's for all NB8B's and SE's.
So any mechanic that deals with NA or NB MX-5's is good for them.
We may be able to point you in the right direction before you even visit a mechanic if you supply some more information.
1. Is the car standard or has it had a few modifications?
2. We may be able to point you in the right direction if you provide a little more detail.
a) Is it at low or high speed?
b) Is it when going over bumps?
c) Is it when turning really tight corners?
d) Is it when going forward or reverse?
Your noise could be as the others said, Diff or Suspension.
Another common one from the rear can be the exhaust swinging around hitting the under body bracing. You can test that out yourself. Just grab the rear exhaust section and see it you can swing it around enough to hit anything.
A lot of SP owners upgraded their rear diff centres to clutch plate LSD's as the open centre was a joke with the amount of power and torque these cars had.
A lot of aftermarket diffs do making clunking noises especially in tighter corners. My one did it when reversing and turning at the same time on my SE.
A lot of SP owners also changed their suspension out for coilovers. Some brands use pillow ball top mounts and these are prone to clunking noises over bumps as they are metal on metal vs the stock rubber mounts.