Rest assured, the RTA are only there to do the emissions testing. they do not care if you come on your own and fail. They are not here to police the laws they are there to provide the services to us so we can make our cars legal.
The police and EPA are the ones you need to worry about. if you get sent to do a IM240 test by police or EPA and you fail then yes you will have problems.
There is NO risk at all of ringing the RTA booking a test and failing. The guys at the RTA like it when they see people go there to try and pass to engineer there car without any defects because they have had all sorts of troubles with fully sick defected car owners getting angry at them when they don't pass because now their rego is going to get cancelled or they have to go and do the test again
They do not even care about the modifications to your car, they will not issue you with a defect if you have a screamer pipe or BOV at the time of the test, THAT IS NOT WHAT THEY ARE THERE FOR!!!
They are there to put the car in the booth test it and move onto the next one.
I have said this many times. Please stop asking this question !!!!