Unfortunately, our current format requires that you ask yourself "How will I fare against LBS?" In this case, because of Lugia, the answer is "not well." Steelix will attack for two turns - enough to take out perhaps a pidgeot and gardevoir, or two gardevoir. You will then knock it out, as well as, if you are lucky, a jirachi on the bench (since both it and steelix are metal). They will then knock you out with lugia. They've thus taken 4 prizes to your three, and they've got all your metals, a gardevoir, and a pidgeot (or another gardevoir, if you don't run the pidgeot). Now, I'm not saying it will shake down that way, but that's the way it will work if your setups are equal. If your setup is bad, it automatically wins. If you get an incredible setup against them, you're still in trouble, because you can't OHKO jirachis with registeel, allowing them to shield themselves while they set up....and that's if you're running the HL registeel. With the other one, you'll get off 2 spins before they ko it, unless they use lugia, in which case you might be able to prize trade with them (OHKO's with gardevoir DS against lugia, assuming a bench of 2 pokemon). However, whereas they're reusing their energies with retriever and power tree, you're losing metals fast.
Also, check your matchups against other very popular decks. How do you fare against nidoqueen? Typhlosion? any liability deck? Machamp? dragtrode? arcanine -ex?
Unfortunately, it is not the time for slow-to-power basic EX'es, particularly if they have bad weaknesses and do not hit for significant amounts of damage. Mercury is a reasonable choice, but a dangerous one.
Good luck with it...