Grey market means a product that was built by company "A" that was never intended or designed to be sold new in a given market. The tractor in question in this thread is one that was (probably) intended to be sold new in the Japanese tractor market. It was never intended to be sold, new, in this (American) market. Shibaura used to make, and maybe still makes, compact tractor for Ford/New Holland.
I wouldn't say that Shibaura (or any of the other greys) made "sub-standard" tractors - they just made them to the standards of their intended market. The most apparent differences are usually a lack of ROPS and PTO shield, neither of which most domestic tractors had for years. Of course there are other differences, usually in throttle linkage, transmission and PTO gears, etc. However, these differences don't necessarily make greys inferior - just different.
There are several name brand tractors (John Deere, New Holland) that are grey market on the US market. They are units that were typically sold in Europe and the UK. There was a dealer in southeast MO that used to import them much in the same manner that the dealers of the Japanese greys import their tractors. I know that JD has gone so far as to issue bulletins to their dealers and took out an ad (?) in Progressive Farmer to warn folks of the grey market John Deeres. Kubota, also a well known brand name, has many grey tractors floating around in the US, although I believe they stopped the importation of their units long ago.
To conclude - I would never rule out the purchase of a tractor based solely on its status as a grey market tractor. The factors of parts and service support, quality, and overall value of the unit would be my deciding factors.
Good luck and take care.