Cheers mate, ther's plenty of 13bt RX3s here in NSW too, and a few MX5s that have been signed off shonkily. The most popular 2 practices seem to be getting the car engineered as an \"injected 13b\" rather than a turbo, and the old switcheroo - getting it tagged as a 12at, rubbing back the 13b on the housings. A cluey copper or the rta inspectors seem to know these tricks though, and either question why there is a turbo hanging off your engine, or get out the calipers and measure the housing to figure it's not a 12AT, but a 13BT.
I'm not sure why they don't just go genuine 12ATs, look at the recent 7 sec pass on small tyres, no 13bt has done ti yet, but a 12at has
so it's not liek you can't make the same power, I suspect it's teh lack of parts, and also some ignorance on the owners parts, there are plenty of big name workshops who will hapilly take your cash for a drive in drive out 13bt conversion on your RX3, and even organise thier shonky engineer to pass it as mentioned above as an inejcted 13b.
Like you have aluded to, an engineer can only sign off on modifications where the statute or regulations specify an engineer can sign off. Engineers do not have the power to legalise something that is not legal, and like you said a few have lost thier right to certify cars here in NSW for passing dodgy conversions.
In NSW (and probably the other states) the regulations specify capacity/weight ratios within which you can legally register a passenger vehicle. If you want to go outside these, the car would need to complied as an ICV (individually constrcted vehicle).
I think the importing of a heavier than aus spec vehicle is legit and a decent way to go here in NSW if you really wanted to build a rotary turbo MX5, and by my assumptions a big set of wheels, hardtop, AC, PS, and you'd be close, if you needed to push it over, why not throw a big MDF box with a big amp and sub in the back?
I've just re-read the rules, and you could actually get a 20bt into a 4 cyl in NSW as long s it weighed 1100kg. I reackon with some creative thinking (not shonky paperwork) you could see your import MX5 upto a weight of 1100kg and legally fit the 20bt. Although I think fitting a 20bt to an mx5, legla or not, it's the owner who would be certifiable, not the car!
The RTA considers a rotary capacity to be 2x it's swept volume.
So for the most common rotary conversions, assuming the car weighs under 1100kg:
20B NA: 3924cc equivalent, anything over 1308kg
20BT: 1570kg minimum
13B NA: 2616cc, 872kg min
13BT: 1046kg min
12A NA: 2292cc, 764kg min
12AT: 917kg min
If the car weighs over 1100kg you could legally put any of the 12a, 13b or 20b NA or turbo varients in.
An engineer can only sign off on modifications where the statute or regulations specify an engineer can sign off, they do not have the power to legalise something that is not legal. They can only fulfil functions conferred on them by others. In NSW (and probably the other states) the regulations specify capacity/weight ratios within which you can legally register a passenger vehicle. If you want to go outside these, the car would need to complied as an ICV (individually constrcted vehicle).
. I know exactly where you're coming from. There's a few 20BT powered RX-3's and RX-7's (series 1 & 2) getting around in SE Qld...all have mod plates and been processed by Qld Transport....however none of them comply 100% with the code of practice and I'm not talking about some minor modification code interpretation issue either....blatant ignorance actually....but at the end of the day what does it really matter? As we know cars are only as safe as the nut behind the wheel. 
