You don't necessarily need a plastic aircraft close to the theoretical limits of radar coverage to reduce the chance of being seen. Word has it that excesive speed or indeed a rather stealthy Tornado complete with big fin and lots of dangly bits can confound the latest systems.
eyethinksy, since when has the position of a transponder aerial had a negative effect on the performance of radar? Call me a bluff old traditionalist, but surely the PSR and SSR processing should be completely separate?