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Let's look at the 3 versions.
With de CAT pipe, how rude!
Round CAT's are slightly smaller and easier to gain clearance, but the oval version can have some trimmed off the, well err, can, if required.
The CAT is part of the mid silencer, we note the tail is a different shape and will not be as straight forward to use the MEV back box as using a MK1 exhaust setup.
This is designed/shaped for MK2 type back box.
The CAT is a separate item on MK2.5's
Noise emitted from the average MX5 with OE exhaust will be around 85 dB. We are allowed 99 for IVA. Most systems will therefore be IVAable (made up word) but note that for track days they often ask us to be quieter than on the way home for some crazy reason, probably the "not in my back yarders" who just moved in next door.