There was a discussion on Defender2 forum where some of the tuning bods were measuring how much power you lost by adding a snorkel.
They sap a measurable number of GeeGees.
That said I have one on mine, but only as it was on when I bougth it. (or should that be brought re another current thread

)
BEWARE however the evil insurance companies. When I spoke to some of the idiot companies such as Admiral, Swinton and Direct Line and mentioned the LR standard exuipment mods on my TD5 when I got to the snorkel they all balked. The line was "But that means you may off road if !!!"
What part of LAND ROVER DEFENDER don't they understand FFS!