Flying an undeclared oversized jib as a spinnaker
Actually, that is a valid way to fly a headsail when going downwind. Its much more efficient than poling out the clew as when going wing & wing because the luff of the sail remains the leading edge and is out of the turbulent air from the main. When the clew is poled out (wing & wing), the leech is the leading edge and this is just plain backwards.
Poling out the tack is an efficient and stable manner in which to go downwind in heavy air if the spinnaker is too much but main alone is not enough or too rolly. It would be faster than no spinnaker at all and maybe easier to manage than a small spinnaker.
The ''modern'' spinnaker actually evolved from this practice.
Try it sometime, you''ll be amazed.
As to the legality of it in a race....I guess it would depend on whether the committee based their definition of sail type on dimensions or implementation. I would tend to think it would still be classed as a 170 Genoa and penalized as such because there''s nothing to prevent it from being used as one, despite claimed intentions to the contrary.