I don´t see how comparison to Sampras is relevant when we´re talking about Courier and Hewitt. Yes, Courier had a losing record against Pete, but Sampras is one of the best players ever on grass and hc, and he was in his absolute prime in the 90s. And as you said, he was past his peak when Hewitt emerged. A prime Sampras would have kicked Lleyton´s ass if they had played on fast grass.
You don't see why we are comparing their success against Sampras? Obviously because he was the best in the 90s on the faster surfaces. Only true way to really link the two players in terms of success as that is the only common element. Even then it is still extremely subjective, but it is much less subjective than trying to point out the field of players, surface speeds, etc.
Courier had his chances in 1991-1993 as he was in his prime while Sampras was still clearly not in his prime, so you can make the case either way. Both Hewitt and Courier faced a non-prime Sampras in their times and both managed to win 1 US Open around the same time with Sampras entering his prime in 1993 and leaving his prime in 2000.