How the Japanese pronounce it is largely irrelevant. It's plainly not designed for the Japanese tongue, even with my extremely limited knowledge of the language.
To me the word sounds like a computer-generated cross between "impress" (which you are supposed to do if you have this car) "her" (to stop it sounding *too* blokey) and the 'z' (however you pronounce it) to give it Italianate flair. Strictly speaking you'd need to 'z's, but there you go.
I've never heard anything other than ImpreTza, from anyone. If they say it different, they're probably not from the UK or Australia or NZ - it's simply one of those words that is pronounced differently depending on your location.
"Brits" (and Aussies and Kiwis) think it's called ImpreTza because that is how it was introduced by the importer - much like the pronunciation of Daewoo changes depending on the country (e.g. Day-oo in the UK, Die-woo in Australia). Any other pronunciation depends on the country you are in. So, please, don't go telling me that ImpreTza is "wrong" and what you say is right
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Andy T