Annoying, but understandable. They don't publish those numbers because the marketing department for the manufactures refuse to give it to them or forbid them from publishing it.
Most concept cars are a styling exercise, which is a subjective matter from a marketing point- performance figures are not. The last thing they want is people comparing the concept car based on numbers to other cars, especially when those numbers are merely pulled out of thin air (with a few notable exceptions, i.e. Mazda Furai- may it rest in peace).
The other reason; and this is true for both the NSX and FT-1, is that when you do a concept car that is based on an upcoming production car, you don't want to publish performance figures because it won't have a positive effect when the production version comes out. If Honda published the NSX as having a 597hp, the press will focus on numbers and people will imagine the production car with those numbers. When the actual production comes out with a lesser hp output, people will be disappointed.