Ok, ganna try my hand at rating this team, however good, no, amazing, it is.
THREATS
-CM Virizion -
If Jirachi is down or even close to it, Virizion easily KOs it with Focus Blast, allwing it OHKO or 2HKO your entire team with its STAB moves and Hidden Power Ice.
-Offensive Deoxys-S -
A combination of Superpower, Fire Punch and Ice Beam, it hits all of your team but Rotom-W for Super effective damage and often a 2HKO.
-Volcarona -
You already mentioned this was a big threat to your team. If it even gets one turn of setup, STAB and HP Rock can lay some serious smack on everything except Gliscor, though after a few Quiver Dances it won't matter with Gliscor's fairly small Special Defense.
-Landorus -
You mentioned this was a threat along with Garchomp, and you're right on that. Garchomp is the only thing that outspeeds it, which can be troublesome, since if it goes down, Mixed versions can lay the hurt on everything save Rotom-W.
SOLUTIONS
Though you have certainly proven that Sigilyph is viable in OU among other places, it can also be considered a weak link in the team and can be taken out if it is forced in rather than coming in on a situation it likes and setting up. Thus, you might want to consider replacing Sigilyph with:
Excadrill
move 1: Swords Dance
move 2: Earthquake
move 3: Rock Slide
move 4: X-Scissor
item: Balloon
nature: Adamant
evs: 252 atk / 4 Def / 252 Spe
This is just about the same thing Fireburn posted a while back, with a few key exceptions. With Balloon Excadrill can stay away from Landorus's Earthquake just enough to get a Swords Dance and do major damage with Rock Slide, which has a good chance of OHKOing. X-Scissor allows you to easily handle Virizion and offensive Deoxys-S (support, as you said, is handled by Jirachi excellently), who could rip holes in our team if they got the chance.
I really, honestly enjoy this team and the read. I hope to face it some time as well! Good luck further on with this!
