exhaust mods

Short answer - if it's not as supplied by the vehicle manufacturer for the model in question, consider it as illegal.

an extract from an ADR bulletin ...

catalytic converters and noise is the main concern ..
so straight pipe exhausts are not permitted but as long
as these bl@@dy Harleys are on the road noise is not
a problem .. lol ..

I have a 4" dual exhaust on my truck with a turbine pipe and
a resonator for noise reduction ... not a problem ..

you can also get permits to remove a cat under certain conditions
but I am not terribly familiar with that ...

but your question was not how to remove a cat ..

if you keep the cat and run a catback system you can run it anywhere ..
there are additional guidelines for vertical exhausts though .

good luck
gmd

10. Exhaust systems
Modifications to exhaust systems are permitted,
providing the modifications do not conflict with
other requirements of this document.
On vehicles fitted with one or more catalytic
converters, all the exhaust gas must flow through
the converters at all times when a vehicle is used on
the road.
If modifying or replacing an exhaust system, make
sure that the catalytic converter(s) and any sensors
are positioned in the same exhaust flow location as
with the original system. For example, some
manufacturers put the catalytic converter close to
the exhaust manifold so that it heats up very quickly
when the vehicle is started. This enables the
converter to function almost immediately. If placed
further down the exhaust system, the converter
cannot function as quickly and will allow
uncontrolled exhaust emissions to be emitted.
11. Noise limits
The vehicle must not exceed the exhaust noise
limits applicable to its date of manufacture. It is an
offence to use, own or modify a vehicle so that it
exceeds the maximum permitted noise level when
the vehicle is driven on the road. If there is any
doubt whether the vehicle exceeds the maximum
limit, it is recommended that it be taken to an EPAapproved
noise tester to be measured.
Maximum noise levels for stationary vehicles are not
to exceed those in the following tables.

dont think it would have one. its only been compulsory an all diesel engines from jan 1 2007....

Can't see why it will be an issue, as long as it has been done by an exhaust centre, and if it's not loud, then who would honestly care.
An Insurance Company might, to try and reject the claim, ( as they will do anything to get out of cooughing up ) but I doubt it.
Cheers
Bucky