i doubt gradius v uses the contra engine. treasure are actually developing the game in collaboration with g-rev, who supplied ikaruga's 3d engine and went on to develop border down, and i think it's more likely that they coded the engine.
and i'm not convinced "treasure box" was ever anything more than a rumor.
edit: and i also think you've got the (apocryphal) psx radiant silvergun story wrong. as i've heard it, crave asked sce technicians about the prospect of porting silvergun to psx, and were told they shouldn't even try.