The Haldex AWD is quite different to the systems in the Subarus and the A6, which are far more mechanical and apportion drive to the rear full-time. Those will genuinely give you extra stability under engine braking etc - unlike the Haldex system, which really is only on-demand, and only to a limited degree. It's also possible to get momentary front-wheel wheelspin under certain conditions, because just like ESP, the Haldex relies on sensors before it reacts.
There are only so many modifications you'll be able to make to an Allroad for performance - it's certainly never going to get to the point where it'd be a handful for even a relatively inexperienced driver unless you're prepared to make substantial changes. The Octavia is a different story - the 2.0 TSI has room for upwards of 250fwkw on the stock turbo if you're keen - but it'll still remain easily controllable in the wet with a bit of prudence - certainly much, much easier to drive than a big-power ute with no weight over the rear wheels, which can be a real handful.
Also bear in mind that you won't be able to fully disable the ESP in any of these cars. Traction control can be disabled, sure, but you won't get any kind of oversteer shenanigans going with any of these cars, without very creative provocation.
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