Okay I am ready for being criticized here.... but.....
From a marketing perspective (I'll put that hat on for now), spectators want/demand and with watch well presented, quick, loud cars that are closely matched. The spec of those cars generally will go unnoticed by many.
From a competitor perspective (next hat please), it depends on what you currently do. Should you already have a TCM car (there are about 5-6), Tarmac Rally (know knows how many), Group S or N (about 60), or a club/road silhouette IROC, you probably all want to compete on some level. Good problem to have to a certain extent if you are an organizer. However the investment for the TCM cars to turn into an IROC is still a large investment, but admittedly less than building an SC up from stock.
Organiser hat now please! As an organizer, you will have interested parties that will have started the process in the first place. Generally these are the likes of TCM owners and ex-rally cars (think Bill Pye's orange 74RS) that would like to run with fellow Porsche's but really don't have anywhere to run other than Marque Sports, and with that category you have modern cars of all makes that will blow the pants off your old 911. You will then have those with IROC road cars that want the barrier to entry lowered so that they can also have a go.
So the difficultly as I see it, is in trying to please everyone. Simply you can't. You have guys (and girls) that have spent mega bucks on cars that are highly spec'd old 911's that are seriously quick. You then have the next rung whereby you have idle cars (such as ex-Tarmac cars) that could be relatively easily modified to suit a spec. No matter which way you cut it, as Hung said, you are going to burn a massive hole in any budget that you try and build to an Aussie IROC spec.
Given the investment, I completely agree with Ryan that you need to have those cars saleable at the end, or all you do is create a new Aussie IROC spec that is near on useless elsewhere.
Love the idea, I would love to do it myself, but fear that I don't have the time or money to play! Unfortunately for me and many others, Group S/N or club sprints will be where our budgets allow us to compete. If you want a 'budget' one make series in a Porsche, this already exists in the 944 Challenge. If you have the means, build/buy an IROC and enter the IROC series – go do it! I hope many do as it would be a wonderful category to have at any historic/state meet. But having yet another spec car that can’t be used anywhere else is short sighted, especially when an international FIA spec already exists, and allows you to take your car overseas and compete against other similar cars.