I've done this with the Heystee kit front and rear. You'd want the Servo off a Deb*nd*r, and a Wilwood prop.valve. Discs will not pull-up with the stock S3 servo. Brakes will be worse. That is why the servo gets bigger on LRs fitted with discs.
It's not about fade etc, unless you live in the Alps or run down Porlock Hill daily? The big advantage is progressive consistent modern-feel brakes, such that - do the miles - and they don't pull or need constant adjustment. They stay 'right'. Don't go'squirrelly' And because they're light, rather than stamp-ON and get ON or OFF, you can feather, have real control. Like modern cars.
The cheaper kits won't allow stock rims, which forces rim-choices which some might say, have your leafer look like it been parked on a sink-estate since the 90s. Done right, gets pricey quickly.
Pointless for showground miles, if you 'drive' and don't just August fake it , hence are found on a wet road in February.... discs are worth every penny.