On regular driving you dont need any cooling air to the intercooler. If you drive over 120 km/h in hot summer days then the intercooler is actually needed. On winter conditions it should be also blocked on the front side or even take it fully out.
Also you can block all the cooling holes in the bumber if the temperatures are under +5 celsius. or if you drive under 100 km/h in summer. On the hottest day you need to make small holes or take the upper block off.
Smooth underside is a also a nice mod.
For tires I recommend to front 175/80R14 Continental ecocontant3
And to rear 165/70R14. This setup need 30 mm lowering kit to front or 60/30mm.. These tires and lowering alone will reduce fuel consumption about 10%
. Check out my lupo thread and Seat cordoba vario threads for more info.
Oh and welcome.
edit: Oh and remember to take out the rear muffler. It should help a lot.