Well my understanding is that the FIA effectively limits braking. They do not want ultra late braking going into corners as that would eliminate overtaking at corners and also be dangerous for a driver following close. ABS systems are not allowed nor any electronic control of the braking process except to compensate for rear ERS. The size of the brake rotors are specified. A single caliper with 6 pistons and 2 pads are also specified. The use of liquid brake cooling is forbidden. So yes there is room to "improve" F1 braking.
And yes I agree with you only jet fighter pilots, A West, and F1 drivers will come close to the limits of this (at this time) mythical car!
Edit: Just remembered:--
The rear brakes are small. Most of the braking at the rear axle comes from engine compression braking. The F1 car is "always" in gear when on track. I believe that small amounts of gas and ignition spark retardation are used to prevent rear wheel lockup when the driver takes his foot off the gas pedal.
We may have had in mind different definitions of "better" braking.
AIUI, currently F1 cars generate in excess of -5g under heavy braking, with the extreme being -5.8g at Monza. AIUI, humans who have not undertaken specialised training can experience blackout as low as -3g. In addition to blackout effects on vision, brain function and, potentially, consciousness, at -5g there is the problem of the weakness of the neck relative to the load being put on it. Most track-day dudes could not last through two laps of -4g braking.
I just can't see anyone except someone who is trained and fit to the same level as a Formula One driver being able to make use of -5g braking, much less braking that is "better" than -5g.
Regarding the potentiality of achieving braking in excess of -5g in the Aston, LMP1 cars generate only about -3.5g. Although LMP1s benefit from much bigger rotors than F1 cars have, obviously the weight is the problem. But how likely is it that the track version of the Aston will weigh as little as an LMP1: 870kg? I'd be surprised if the Aston came in at under a ton (apples-to-apples with the LMP1). Without an air-brake of some sort, there is no way that a car weighing a ton could generate in excess of -5g, when the best a car weighing 870kg can do is -3.5g.
Where I was coming from was that I am sceptical that the Aston will be able potentially to generate more negative g than a Formula One car can achieve and, even if it could, the amateur behind the wheel could not cope with it.
If however by "better" you mean modulation, control, and the ability to exploit the braking system, you make a good point. ABS helps a lot on a circuit. Unless one has the leg strength of Ezekiel Elliott, servo assistance is necessary with either carbon-ceramic or carbon-carbon rotors. Keeping the brakes (and of course the tyres) within their designed operating temp range matters in F1 and LMP1; doing that in the Aston will be another engineering challenge, although I can't imagine that brake cooling should be a problem.