Those lap time or total lap comparisons are pointless because the track is hardly the same. I'm not even talking about the chicanes, but mainly the Dunlop Bridge section being replaced with the slower esses and numerious other small changes to the track, which some could have made it actually faster but overall it is a slower track now.
Btw, 787(B) never ran on the full straight. Chicanes were built after 1989. 787 debuted in 1990.
Also if you want to compare GrC to modern LMPs, you should probably pick a faster GrC car... more about this below.
They didn't explicitly ban rotaries. They just changed the rules so that the teams had to use Formula 1 engines (3.5 liter) to compete. Everything in an effort to get the continental Europeans back on the podium. The English (Jaguar) and Japanese (Mazda) were effectively shunned from Le Mans due to Jag's 7.3 V12 and Mazda's 2.6 R4.
This. The "rotary ban" is a complete myth based on false beliefs. I wouldn't be surprised if this whole myth started in the internet age. Everytime I hear this myth repeated it seems to include the assumption that it was banned due competitive advantage. That is so wrong in many levels: firstly of course factually, but it also undermines the other manufacturers who actually dominated that era.
Just adding to what you said: the rule changes were in already sealed in mid-1990 and thus likely been in the planning phase for some time. Certainly, the changes was already known when Mazda won (reminder: in 1991). In fact, 1991 was already a transitional year between the old and new regs. In addition, Mazda never showed any sign of domination with its rotary engines even before the 787(B). Besides Le Mans win, 787B couldn't finish on a podium in the Word Sportscar Championship in 1991 and ended up finishing 5th in Championship. Even in the race [Le Mans] it won, it wasn't the fastest car in outright speed.
The rules in 1991 were messed up due the separation to "C1" (new cars) and "C2" (old) categories and this actually helped Mazda. Autosport article from 2011:
And Le Mans was an anomaly. While Peugeot ran its usual 905 contender in a 24-hour race for the first time, Jaguar and Mercedes both preferred to fall back on more proven machinery. The Jaguar XJR-12 (the winner in 1990) and Mercedes C11 (successor to the Sauber C9 that triumphed in 1989) both had to run ballasted to 1000kg, compared to the 750kg of the 'atmo' cars. Since Peugeots weren't expected to last, nobody at Merc or Jag saw this as a major problem, since the older cars would be fighting each other on equal terms.
There was some small print, however. A bit of politicking from Mazda caused the limit for rotary-engined cars - and who else could that be? - to be pegged at just 830kg. Nobody paid too much attention, but Mazda knew that having to carry 170kg less than the ostensibly faster cars for 24 hours was going to be something of an advantage.
Well, that probably wasn't the only reason for the win, since most of the top contenders DNF'd, but still.