1) What is the best induction kit/panel filter for actual power gains.
2 schools of thought here, some will say get a good cone filter with a cold air feed, Pipercross and Green are popular, others will say a decent panel filter, with a drilled and smoothed airbox, bit of an open can of worms this one.

2) Please can you point me in the right direction of part numbers for larger ARBs? I’m assuming the FR models have thicker bars and that would probably be a decent step in the right direction.
If you are looking to buy new, go for a non OEM one, some of the site sponsors will help you here, (Awesome GTI etc), best to spend the money only once, get it right first time.

3) Do any of the VW Golf GTI parts fit the Leon? Would any of these offer greater improvements to braking/handling than Seat parts?
Golf (mk5) GTi parts would be similar to Leon FR or Cupra parts, so yes, there would be common parts, but not necessarily better