It is really hard to judge the ability of old pros hit until you get on court with them. It's usually quite silly to try and determine someone's ability by who they have beaten, and then subsuquently who that guy has beaten, and so on and so forth.
But that's not going to stop me from trying.
Last year I got the opportunity to hit with Agassi, Chang, Courier and Martin before an event. A lot of it was just messing about, but I hit with Chang for a minute or so and it was completely different to anything I've ever experienced. He was a complete backboard. A machine. His ball was so clean, constant, and deep.
I like to tell myself (an average 5.0 player from the ground) that I held my own. I even "won" a couple points. But deep down I know he was taking it very easy. I feel that if we played a set with serves I could maybe get a few free points off my serve, but they would be few and far between.
I've also hit a few times with TonLars on this board. He is a 5.5+ player who plays In a few satellites. We played seriously once 2 and a half years ago and he cleaned my clock 0-1. The games weren't super close, we maybe went to deuce 4-5 times. I came away feeling that if I wasn't so nervous (50+ people watching) I might have served better and at my very best won a couple more games.
The thing that sticks out looking back at this is that both players (Chang and Larson) is that both were extremely impressive and daughnting to play. The difference between the two was I felt I had a chance to win points and even games against Larson, whereas during my brief hit with Chang I felt as though I had no chance. I was at his mercy, a puppet on a string.
Alright time to attempt to answer the original question. Chang and McEnroe are obviously not the same player, with differences in playing style, age, fitness, and # of grand slams won. They were both at the top of the ATP tour at one point in their careers. I would take a punt and say right now they are of similar ability. Hopefully that gives some insight into how skilled these guys are.