An EV enthusiast - temp (@tempermanant) via Teslarati - notes several differences between the new 2021 model year version and the previous 2020. Documents for the Tesla Model X Plaid are not yet available.
As it turns out, the car's curb weight is about 5% lower (5,219 lbs/2,367 kg vs 5,498 lbs/2,493 kg) than the previous Long Range version. It's 126 kg less. By the way, the new Tesla Model S Plaid is also lighter than the previous top version.
The battery weight reduction is 14% (from 625 kg to 537 kg), as the energy density improved 13% from 165 Wh/kg to 186 Wh/kg. That's a solid 88 kg less.
As we know, the refreshed Model S/Model X have fully redesigned modules and battery pack (see here). This allowed to increase the pack's energy density by 13%. The new nominal system voltage is 410 V (nominal) compared to 360 V previously.
Another thing is that the new Tesla Model X has a third more power and currently both the front and rear motors are Permanent Magnet - Synchronous Reluctance Motor - PMSRM. Previously, the front motor was an induction type.
The front motor is rated at 243 kW, while the rear at 248 kW, compared to 180 kW and 189 kW previously. The battery capacity appears to be around 100 kWh, but there is no direct info about that.
Direct links to documents: