Most importers don't know any more about the car that they are selling than what's on the auction sheet and anything that they can see with a quick visual inspection. There are exceptions to this where the importer buys direct from the tuner like the Endless drag car mentioned on this forum recently, but those are rare.
My car came from a reputable importer, was described as having been "prepared by Top Secret", and the engine was dressed as a 'Top Secret Competizione R' which a little research showed me should be a 2.8 bottom end.
Needless to say the polished engine cover and plaque turned out to just be dress-up.
Unless you have the paperwork to prove it, and have spoken to the tuner to cross-check, you'd better assume it's standard. Even if you do have paperwork and have spoken to the tuner the previous owner may have swapped the engine for a stock one before selling it.
If you're buying a car from a Japanese seller - which many importers are, claiming no responsibility once the car leaves Japan and saying what you assumed was their 'UK agent' is nothing more than an unconnected businessman who makes money doing SVA conversions - you have no comeback, and no resort to the UK Sale of Goods Act (which enforces the seller to provide an item as described in any advertisement).
Buyer beware!
My advice: buy from a person on this forum, where the car is in this country, can be inspected and has a known and verifiable history in the UK.
P.S. You may not have heard of the nightmare stories because they've been buried, or removed from sites due to libel action threats. Do your own extensive research before parting with the most amount of money you are ever likely to spend outside of buying a house!