The deckplate mod was Robert Cannon's (of Frankentaco fame) idea in furtherance of Gadget's discovery of the elbow-mod. The elbow mod is free - just remove the elbow in the fenderwell area (note - DO NOT remove the "S" shaped tube that plumbs directly into the airbox). The deckplate costs about $10-$15 and requires some work, but supposedly acts to better protect the intake from any accidental water intake - although it's obviously not waterproof and nowhere near as secure as a true snorkel.

As far as performance goes, some argue that it sucks in hot air from under the hood, much more so than the elbow mod which sucks it in from the cooler fenderwell area. Others argue the hot air, if any, has no negative effect. Gadget is the only one who has proven dyno numbers on his website, but they pertain to the elbow mod, not the deckplate mod, at least based on the last time I checked, which was a long time ago. His website is at www.gadgetonline.com.

Unfortunately, I have no personal experience with the deckplate mod, so I can't give any firsthand accounts. I've done the elbow mod on a 99 2.7L I used to own, but no dyno results to show whether I gained any hp from it. But given Gadget's dyno results, I would think I got some minor hp out of it.

Hope this helps.