I feel that Gretzky is the player in history who had the most positive impact on teammates, above Orr. Gretzky on Kurri, and others, etc. So - as much as Orr would help Gretzky, the opposite is also true with Gretzky helping Orr, and Orr would produce more.
I didn't get to see Esposito play - but it sounds like he was supremely talented and a great compliment/finisher to Orr. Do Lemieux/Gretzky do better in that role? Sure, maybe a little bit since they're both more talented then Espo, but we wouldn't be seeing 300 point seasons.
I think the biggest difference would be on team results, vs individual statistics.
It's true on the 80s oilers, a peak/prime Orr playing with Gretzky may help Gretzky score up to 230-250 points in a career year, vs 215. But the biggest differentiator would have been come playoffs, and in winning games. Oilers lost the cup in 83. and in 86. Maybe even 82? It's possible with Orr/Gretzky combo they win more.
Same idea with Lemieux. The effect on Lemieux would have been a bit bigger early in his career, since he was slower to start. I think Lemieux's point totals can shoot up prior to 89 if he got to play with Orr. That team would obviously be making playoffs too - and possibly competing for more cups before 91. I'm sure Orr could have helped push Pittsburgh to the finals in 96, maybe 93. So again, team success, it helps a ton. Personal statistics? Same as Gretzky. Maybe in a career year Lemieux could approach that 230-250 range (again, without healthy seasons which he lacks, it isn't exactly happening) - but you wouldn't see 300 points, no.