Sliver Overlord isn't always a better commander than Sliver Queen , they both have their place in the game. Sliver Overlord is great if you want to consistently hit the same creature combos every time you play, Sliver Queen is great if you want to roll the metaphorical dice and see what abilities you can get out and then just populate the hell out of the field.

I would suggest looking into Eldrazi Monument , since you can put out cheap creatures fairly consistently late game it wouldn't be a bad idea and it would save you from board wipes, even your own.

Also, I would suggest Coat of Arms over Door of Destinies , it works instantly, rather than having to wait for you to cast spells.

In terms of planeswalkers, check out Garruk, Caller of Beasts . If you can ultimate hit with a deck like this, you pretty much win.

Finally, Dementia Sliver is really not that good, especially in a singleton format. Swap it out for something to either boost your guys like Might Sliver or one of the trample slivers (which I think you should have anyways) Horned Sliver /Groundshaker Sliver

PS. Invest in a better mana base. It's too small to start with, and you're missing some things that are pretty important. Cavern of Souls should be an auto include, along with things like Crystal Quarry , Reflecting Pool , the shock lands, fetches and ABUR duals if you can afford them, and once you have your colours more readily available you can throw in some more utility lands. Things like Haunted Fengraf , Maze of Ith , and Reliquary Tower .

Your suggestions are much appreciated!

I have revamped the deck now, including the cards i do not actually own - for tappedout.

The previous problems were simply owning the cards and or budget :)