UK computerized emissions testing requires VIN to be entered, at which point, entering the Roadster VIN will cause it to be not recognised.
If they are testing a Eunos Roadster as a MX5, and then telling you its failed, I can only assume they are trying to spoof the system uing a VIN they know works.
Its irrelevant whats on the logbook/v5. Some testers will insist, until blue in the face that its a MX5… Which funnily enough is the opposite when you try and buy parts from Mazda.
And decatting makes no difference. The MX5 used a high flow rate cat when it came out. As pointed out, combined with a standard exhaust, decatting results in an Italianite rasp. Decatting when you already have a loud exhaust leads to a random outcome.
Plus its just not PC. We no longer throw rubbish in the streets, let dogs do their bits on the pavement. Most of us will happily sort our refuse for recycling, composting etc. But its ok to remove a Cat, something that was fitted to help improve environment. And its no longer a case that a cat, for an old MX5, is really expensive to replace compared to fitting a test pipe; the costs are similar.