Hi mate. The Leon is very tunable particularly the 1.8T engine which is a 20 valve VAG (VW/Audi Group but you knew that!) unit.
A remap will take power typically from 150 to about 200 bhp and this should be reliable power. If you swap and uprate the turbo as well you can reach around 300 bhp but ideally at these levels you need to get the engine rebiult with more solid internals.
Adding a sports exhaust and induction kit will make the car a little more responsive. Dont fit a BOV (blow off valve) as I've heard people with them have had problems with the ECU getting confused at the sudden loss of air in the VAG units.
Lower the car 30mm and fit uprated shocks. Get a big brake disk conversion with some sports pads and you have an awesome track day and everyday car.
Hows that for starters (I think I'll do an article on the 1.8T engine as it is a personal favourite of min
Especially since this is the unit in my A3 T Sport!)