Usually stay away from these discussions as it is near impossible to compare players in different eras.
That being said JS was probably the most difficult ND receiver to cover man to man I've seen in person (aka 1961 on).
-Like another poster was at his first game, Purdue. After seeing him in the Spring game vs a good secondary could not believe that Purdue covered him man to man - they took a somewhat different approach two years later.
-Still remember a game vs Oklahoma: the ball was inside the 10 with JS lined up on the short side of the field ... we ran a simple drop back out pass ... JS ran his pattern so well he was open by nearly 10 yards ... his defender didn't fall down etc JS just left him well towards the middle of the EZ.
-Only saw one team actually go man to man once everyone knew how good he was: Purdue. They used Leroy Keyes as a DB with a lot of effectiveness. Not sure that would have worked so well had ND been able to study film of him doing that.
BTW Eifert might have been the best receiver "ever" for an offensively handicapped team. In 2012 he was our passing game and every team knew that going in. Was amazed on how we schemed to get him a chance to get open and what he did with those opportunities.
A little off topic ... going into 1966 we had the four best receivers I've ever seen on any ND team. Two are well know JS and Bob Gladieux, two not at all: Paul Snow (Jack's brother who hurt a knee and only had a brief career but a spectacular start in 67 until he reininjured it ) and Curt Henegan (he was running neck and neck with JS in the Spring but hurt his leg offseason and came back a step too slow so again had a limited career)