Since the OP has his query answered I thought I would reply to this.
A few moons back I started a thread on better brakes and come to the conclusion that the best return for your £ was better tyres (Click
here). The autoexpress article demonstrated the difference in braking distances with the better tyre. Looking around for some better 'material' these 2 popped up...
http://forum.miata.net/vb/archive/index.php/t-260280.html
http://www.edmunds.com/how-to/how-to-choose-tires-and-wheels.html
Since I have / had a little soft spot for the MX5 I remembered that another benefit of the 15 inch is
handling and it's not surprising to see that the Miata's standard wheel is now 15 inch.
There appears to be less in the way of availability of performance tyres in 14s (even 15s are not great compared with 16s).
When I looked at the actual brakes on the 500 the ABS worked differently on ones fitted with ESC. I would have to dig out the actual technical differences but I got the impression that on the 1.4 the brakes worked quicker / better since the ESC is a standard fit plus it has discs all round with the fronts being ventilated (the A500SS are also cross drilled

but that's more for show). So if you have a really good set of tyres on this setup you would probably get reduced stopping distances.
I've only driven a 1.2 - never owned one so I haven't got to experience the limits of its brakes - so I would have to take your word that the bigger foot print would make no difference.
EDIT: Found that 'technical' document on the differences that ESP makes on the Braking on the 500 - it is only in the area of Emergency brake assist. The opening article states that
Vehicles fitted with ESP also benefit from hydraulic brake assistance but then goes on to state that if ESP is not fitted it will still operate with equal efficiency...
http://www.carpages.co.uk/fiat/fiat-500-safety-19-01-08.asp
On Fiat 500 versions without ESP, emergency brake assistance takes the form of Emergency Valve Assistance (EVA); a mechanical system located inside the brake servo. On vehicles fitted with ESP, the function is carried out electronically by the ABS control unit and is known as Hydraulic Brake Assistance (HBA). Both systems carry out an identical function with equal efficiency.