[Matt F]

That link is talking about remapping which, IMO, is the way to go - much more sophisticated than a chip/plug in performance box.

I've heard Celtic Tuning are very good in this respect and will remove the map and refund your money if you aren't happy with the results.

And if the warranty invalidation bothers you, try Superchips - their conversions cost more but their own warranty covers any faults that may occur.