I heard that as Jamie says Honda were so pleased with the NSR factory bikes that they made the street bike (MC16), which was in turn so good that they basically used a modified street NSR engine for their production RS250 GP bike in a more racey chassis, Hence why the RS250 has a blanked off 2T oil pump. The only people who have tried using a street based engine in Grand Prix since then that I know of are WCM (allegedly...maybe....so it was said).