Always drive it downhill..............
If you really want to drive fast, why did you choose the 4x4 ?.There aren't enough of them around for us enthusiasts, without 'racing-drivers' wrecking them.
However; if you want to go faster off-road.....that's different. Allways go for the K&N filter, but forget about drilling holes in your bumper. Take a hose from your air filter 'UPWARDS' as high as you dare, ensuring that a, it dosen't scoop in rain, muck and hot air and b, if it does, the results can be drained off before reaching the carb.
Fit M&S tyres...or pure off-roaders (75% +) if you are trailering the car.
Fit a re-usable, easy-clean, in-line fuel filter.
Obvious things like:- ensure that your timing is spot-on as well as tyre pressures, wheel-balance, etc.
Forget rubbish items like spoilers, etc., you are driving a house-brick !. However, as the owner of the 4x4, with a little practice, you can virtually outdrag almost anything for the first 100 feet..........and they are the one's that matter !
Enjoy your Panda !