I use the SLX3 in a 40cm width and find the brake hoods to be about 13mm narrower than the 40cm c-c width at the ends. I can't help with the Aero model.
As for the reach, it would make no sense to use the same stem length with short reach bars unless you're intentionally trying to reduce the reach to the brake hoods. I swapped to shorter reach bars so I could move may saddle further back and keep the same reach, with the same stem length.
As for the saddle to bar drop, you might find that more drop and less reach to be an acceptable trade off, but I like to evaluate them separately. I like to have enough reach so I avoid knee to arm contact when I pedal with my hands in the hooks and my upper back is in a low, aerodynamic position. To get a low upper back, it either requires a substantial saddle to bar drop or a lot of bend at the elbow. A lot of bend at the elbow will create knee to arm interference quickly. I prefer to use very little elbow bend and more drop, just like a lot of the pro riders. A couple of years ago I increased my saddle to bar drop from 9cm to 11cm, but made no substantial change to the reach. This can be tough to do, since removing 2cm of spacer also move the bars forward by 6mm. I did it by changing from an 84 degree stem to a 73. This still increases the reach by about 2mm, but that's not nearly as much.