Interesting to read different responses here.
I have the TJM snorkel and have experienced no performance changes. I have no doubt that the snorkel will induce extra drag just by virtue of its existance. But nothing I can measure a this stage at normal road speeds (100+).
Son has the Safari on his TD5 and doesnt believe he has gained or lost anything either.

Both vehicles happily go beyond 4 grand without any hesitation.

The onlymeasurable changes on these 2 (aside from water crossings) is the cleaner filter and cooler air.

cheers