I still have the stock exhaust from my 93 v.spec; I had it measured when it was on and IIRC it was 87.something dB at 4800RPM.
There's a massive difference in audible sound level between 88dB and 96dB - maybe your mufflers have packed it in?
Was your test administered by the EPA or by the mechanic? If mechanic, visit another and get a test. Reason:
* dB meters can go out of calibration (most exhaust places don't calibrate; the EPA is supposed to)
* the test has to be administered a certain way (many exhaust places don't know the method, it's in the ADR)
* there is a temperature correction (IIRC, for temps different to 25 degrees). It is very cold now; low temperatures == louder, most exhaust places won't correct.
* there is a frequency weighting curve (don't know whether meters apply this automatically or not).
Don't really know. I don't think that 94dB is what a stock 90 GTR came out at; but since I don't have one of those I can't tell.
Since it's post 1989 (Motor Vehicle Standards Act) you'd think that it would have had to comply with whatever the emissions requirements were at the time.
You need to find out which ADR your car needs to be compliant with.
If it was Aust-delivered or imported pre-1992 that would probably be ADR28/00 (93dB max limit, 92dB stated but within 1dB is compliant, drive-by test).
If 1992-2005 might be ADR28/01 (91dB max limit, similarly 90dB stated but within 1dB is compliant, stationary and drive-by tests)
http://www.fordforums.com.au/vbportal/modules.php?name=Content&file=viewcategory&cid=18.
has what looks to be the originals of many ADRs
Cheers,
SW