You could lose the Psychic Trances and replace them with Disruptive Pitmages, which do the same thing for less mana. Also, since you're running blue, consider adding some stasis elements such as Frozen Aether to slow your opponent down even more. And if you're relying on Phage as a win condition, you may want to increase the number of her from 2 to 3 or even a full playset in the deck. Also, a nice combo with her is to put a Whispersilk Cloak on her...something to think about. :)
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Look into Sound the Call, and also Tel-Jilad Wolf for some artifact protection. Consider too, that the Packmasters have to champion an elf when they come into play, so you may want to throw some llanowar elves in or something that they can replace other than Tolsimir to get them out more quickly.
Consider replacing Shadow Sliver with Shifting Sliver. Shadow sliver makes it so that your hive can only block creatures with Shadow, whereas the Shifting Sliver will make it so that your creatures can only be blocked by slivers, while still being able to block in return when your opponent attacks. And if you combo that with Brood Sliver, you'll have tokens a plenty. :)
Is Blazing Archon supposed to be in there? I don't see any means of white mana production in here, unless I'm missing something...are you discarding it, then bringing it back?
From looking at what you have there, you could lose the Mindstabs (I see why you have it in there, but it's a little casting cost prohibative), the Delays, and possibly even the Evacuations, as well as a couple of the lands. The Lotus Bloom with all the counter removal things you have in there will give you ample mana, plus if you cut down your overall number in the deck, you won't need as much land to have it go off. Hope this helps!
Really a pretty decent standard sliver deck. The only thing I might suggest is going without the Whitemane Lions and instead throwing in a Sliver Legion and a couple more Virulent Slivers (the toxin counters will stack then). Very nice overall!
I'd agree that the addition of gemhides made making a multi-color sliver deck much more feasible. I worked for over 3 years on my rainbow sliver deck, and just NOW got it to the point where I love it (mostly due to those gemhides!). You've got a good start to a two color sliver deck here, but might I suggest a couple essence slivers to give you some life? Also, a two-headed sliver might be a good addition on the red side. Overall tho, looks good!
Thanks for the comment - The Root Sliver is actually in the sideboard for this deck, which I haven't posted yet, as is the Sliversmith. I know about the shadow - and I usually only play the Shadow if I have a Brood in play and no Shifting or Winged to get the damage through.
Just as an fyi - I don't play this deck in tournaments, just casual. So having more than 1 Demonic Tutor is irrelevant at this time. If I decide to take this deck to Vintage, 3 of them would be replaced with the cards mentioned (i.e. Diabolic Tutor, Vampiric Tutor, etc.). I'd also like to add an Undead Warchief or two, but they're not in my collection right now and have to track them down. Thanks for the input!