I have a thought on the way this deck is going...

It seems to me that its closest relative is Threshold. It is one of several Threshold-esque decks running around these days. Theshold is in three (or four) colors because its creatures are Green while all the actual good cards are in other colors. Don't you think that Threshold would be playing two colors if it could? Consider the following:

1. The mana base would be easier to smooth out in two colors leaving room for something like Wasteland or Ancient Tomb or whatever.

2. If you guys are going to play three colors, it should be UWR for maximum exposure to the same cards Threshold wants to have - Lightning Bolt, STP, Fire//Ice, etc, not for more creatures.

Bottom line, the hard work for this deck is done. It was done with Threshold. Keep the U and W Slivers you already have and if you look to a third color it is only for the control elements that we all know.