I bought an 18" excellerator for my DFX in '04 or '05. got a good deal on it through ebay... about 1/2 price. It definitely didn't perform as well as i hoped, on the beach or in the parks. the depth wasn't as advertised, pinpointing was literally HELL, and the added weight...
I ended up putting it back on ebay after about 2 months, got more than i paid for it originally (I had used gravediggermax's epoxy trick for the bottom of the coil", and bought a 14DD coil directly for whites. I'd pass on the excellerator. Go with a coil made by the factory for your machine
steve