With no experience of a Signum, I cannot comment directly.
But on a ST the Bluefin did wonders. Ok it was FWD, but manual. Quicker of the line, definitely more power and faster around Brands hatch, Snetterton and Cadwell (Bluefin and Dreamscience). Similar experience with a 335i with standard auto.
That was just us driving and using a simple timer. No fancy racelogic box etc. In fact I would question why straight line acceleration would be a useful marker to any of use who don't have the use of a so called private test track with a straight stretch of road.
So not snake oil at all. Remember there might be issues with your car that you are not aware of that the remap may be exposing. Is your clutch upto scratch? Some of the ST owners had issues and we eventually identified that they were suffering from clutch slip and once the clutch was upgraded they were able to appreciate the benefit of the remap. Alternatively it may be that the Signum is not capable of handling a power upgrade (transmission, gearbox or tyres). I suspect if they were selling snake oil they wouldnt be offering a full refund policy.