Edit: Yes, that's custom carpet to cover the painted metal exposed by removing the rear seat bottom & the hatch sub-floor.
Ya, only a tiny improvement in acceleration (and braking and cornering), but it's a free performance upgrade

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You don't get anything back in regen below 7mph.
In the city with the seats left in, after using extra power to accelerate the extra weight, you get no extra regen from that weight below 7mph, yet it does add a little wear to the brake pads & booster pump, and the latter also uses a little more power from the HV battery.
Some drivers won't always be able to do this though: I prefer the security of leaving no valuables in the car. With no seat, any would-be thief can clearly see there's no reason to break in

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