Group 1 had some serious work done. Before, you couldn't even break 300kmh with any hybrid LMP, and you needed to gimp the downforce completely to match their real top speeds at Le Mans. Now they have less stock downforce (1100/1400 from 1200/1800, Group C has 750/1500 from 800/1400), MUCH LESS DRAG, and they're easier to drive as well. The GT-R actually has a worthwhile speed advantage now.
That being said, the gimped the hybrid motors H-A-R-D. It's practically impossible to drain the batteries now, but the cars (especially the R18 2016) aren't dragsters anymore. Narrower deploy range as well - Porsche 919 deploys from 95-235kmh, the TS050 from 83-240kmh, and the R18 from 86-239kmh. Cars with small batteries, like the McLaren, TS030, and L750R, no longer deploy past 250kmh. And yes, this is part of an effort by PD to actually balance this category. Does it mean Gr1 races will no longer be 919lol or 787Blol? Probably not, but it's better than what we had.
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Oh, and they deploy while turning, so that's cool