People put too much weight on accomplishments like "career Grand Slam." It is a great accomplishment, but how do you weigh it? For instance, how does it factor in comparing Agassi with his career GS but only 8 total Slams vs. Sampras with no career GS but 14 Slam titles?
And whether you win all four or four of one, both mean that you played well enough to win a Grand Slam event four times in your career.
I think at most you can give a player a "+1 Slam title equivalent" for a career GS, so Agassi's adjusted Slam count--in terms of comparing him to others--would be "9" and, if Stan won Wimbledon, he'd be at "5." But even so, you have to look at other factors. Would Stan then be better than Courier and Vilas, or even Murray? And was Agassi greater than the two other 8 Slam winners, Connors and Lendl? I think not. In fact, I'd rank Agassi behind them, and probably also behind McEnroe (7).
My basic point is that ranking players is complex, and we need to go beyond Slam count and such things like career GS. We also need to look at other titles, rankings, non-win results at Slams, etc.