I'll get the ball rolling with R03 car details! I'm a few days ahead of schedule which is nice.
To give a bit more detail on the changes front to back.
Front Wing: Low drag configuration (plus end plate changes) of R02 FW. 15% less downforce, 30% less drag.
Front Wheel Deflectors: Evolution of design from R02. No inherent performance on part but it enables better performance downstream.
Floor Fences: Position of fences adjusted to correct CoP of the car and adjust size/location of main floor vortex.
Side Pod: Cooling section as R02 but differs in blend to new engine cover. Louvres adjusted to bring cooling closer to target.
Engine Cover: New part, incorporates walls to capture and handle cockpit/halo losses while delivering better quality flow to the rear wing.
Rear Wheel Winglet: New design focused on encouraging diffuser lateral expansion and rear wheel wake control.
Based on my current correlation (but still quite speculative compared to an official run) I am expecting to see:
Cl: +10%
Cd: -3%
Cop: -1% (Rearward)
Cooling: -3%
Which equates to 1.1s a lap faster around Monza compared to R02 spec car.