i recently fitted my Seat Sport strut brace and it made a big difference on my FR TDi. A lot less understeer going into bends and around roundabouts, can now feel it grip into the turns (you can even ask my gf who was pretty amazed at what difference it made)
However, the FR's do have slightly higher and softer suspension compared to CUPRA's so don't really know about the impact on them.
RichieRich > you wouldn't feel much on your Leon due to it being overall heavier and more grippy naturally (i have noticed this with my cousins Leon FR)