Do aftermarket or upgraded capacitors make much of a difference in spec classes?

I sugest to use the search because there are so many topics about capacitors and optional ones.

It is minor the biggest difference can come from the actual esc. Many people assume that as long as the specs can handle the motor that the cheap spec esc’s are nearly as good but simply not the case

nope.

capacitor only helps with voltage spike just like you use them for the high wattage car stereos, having good clean solder connections probably make a bigger difference

A capacitor stores current to reduce the spike when a load is applied

On DC voltages yes, but these capacitors are too small to do something in that case, the low resistance of the battery is capable to give enough power w/o high power drops. The capacitor is there to filter the small high frequency AC part on the power line caused by the switching FET's This high frequency ripple can make the FET's of the ESC and also the battery hot.

I believe MonacoRC has a non polarized capacitor. This is a small PCB with many ceramic capacitors parallel. Ceramic capacitors have a much lower ESR than the normal used electrolytic capacitors and with many parallel the ESR will be much lower. And if you want to have a next better step then a capacitor bank of tantalum capacitors is the best choice but expensive. Well, not that expensive to try but for the manufacturer not a good choice in costs compared for what you get (more costs for nothing noticeable)

Any gain? No, unles you are a guy winning many worlds prooving you can get the best out of it. Then maybe with a better capacitor you can get 0.05 sec a lap more out of it....

Even then low ESR in practicality only comes into play at vhf frequencies. ESR does effect EMF of a capacitor some what in the sense of dissipation in a RC filter. Capacitors in parallel with the battery effectively have a even lower esr since the magnitude of the battery vastly out weighs the capacitor. However the high frequency that can ripple on the DC can dissipate in be battery or esc and this can be sent to a RC filter or a capacitor in general with a specific internal resistance to dissipate interference as heat. Attenuating Voltage drops and spikes with capacitance on a battery where it is mostly DC and the resistance of the capacitor is near infinite then in this case the ripple current of a capacitor is desired.

I would be more inclined to reduce VHF ripple or feedback, and the RC circuit can be very specific if one knows the frequency hence where a nice oscilloscope is handy.

I did a small test (not yet on a car) and made a capacitor bank from ceramic capacitors. The bank contains 9x 100uF-16v ceramic capacitors, these are not polarity sensitive. Why 9? Looks close to 2x 470uF normally used.

At work we have a capacity with ESR meter and the capacity is 870uF and the ESR is 27mOhm ! I looked in the specs of a normal 470uF capacitor which is used on the Euro spec Fleta ESC, the normal has an ESR of 220mOhm and a low ESR has 170mOhm (not measured but based on some specs I found).

I will give it to someone to give it a try, I highly doubt he will notice a thing but maybe a cooler ESC and a much easier mounting.
On the pic a comparisation of the size of 2 made banks compared with a 470uF capacitor.

Looks nice. I would be willing to bet if I had money to spare that it indeed would reduce esc temps.

you have try to assemble more ceramic capacitors ? and you have tested the result?
i think the best advantage is the low esc temperature therefore more efficient . Also i have see that many rc manufacturer use cheap capacitors for assemble power bank , i have order 680uf 16v Panasonic EEEFP1C681AP low esr instead of 470uf and i want assemble a bank with three of these.

The 900uF capacity is equal with most capacitor banks using 2x 470uF

The test results were promising. The guy testing this on a modified touring setup gave him a stronger dragbrake and normal brake by just swapping capacitor w/o any reprogramming of the ESC. With some practise rounds he did set a new personal lap record. So yes, it does have some influence on the ESC.

More is possible, the PCB is dubble layer so can mount on the other side as wel

if the coils and rotors are designed around a trapezoidal waveform (they are) anything that sits outside the waveform turns to heat. cleaner power means more power. how much? I dont know but it should be easy enough to load a motor up and watch the waveform.

more uf capacitors are always better ? which is the better capacity in uf for an esc 2S stock spec ?

They make a difference on my wallet, lol.... But in all honesty I run what is installed on the ESC, in this case HW XR8 Plus. I have not found any issues / glitching or other with what I have. Nor the need to spend money, case in point that guy offering "Drag Special" Cap packs....

Maybe some of the diehard 1:12 guys may have some insight on their end.