You have 5Q0511115AN rear springs on your car already. These are the stiffest Octavia MK3 Scout rear springs.
The Octavia MK3 Scout 5Q0511115AN rear spring has the same specification as the Octavia MK2 Scout 1K0511115CT rear spring.
I've listed the below springs soft, medium, and hard so that you can see the hierarchy.
The hard spring is about 10% stiffer than the soft spring, whereas the medium spring is about 5% harder than the soft spring. So they are actually quite close in stiffness.
Octavia MK2 Scout rear springs that are equivalent to Octavia MK3 Scout rear springs
Soft
1K0511115CR is the same as 5Q0511115AL (Wire Diameter 12.5mm, Length 378-380mm, Outer Diameter 114-115mm, Weight 2.4-2.45kg)
Medium
1K0511115CS is the same as 5Q0511115AM (Wire Diameter 12.75mm, Length 382-383mm, Outer Diameter 117mm, Weight 2.6kg)
Hard
1K0511115CT is the same as 5Q0511115AN (Wire Diameter 13mm, Length 382-383mm, Outer Diameter 117-118mm, Weight 2.75-2.8kg)