Time Walk I would argue is the most broken card in magic if not fighting for the slot with Ancestral Recall, Black Lotus, and Yawgmoth's Will. Not only does it mean that you are essentially playing the 59 cards but it gets a whole another land drop (huge in landstill) and a whole another turn.
There is NO reason to cut Time Walk, ever.
To me, it is very important for Landstill to focus on cards that bring answers. Brainstorm is fine for mana smoothing and deck digging, Impulse is VERY GOOD at deck digging and mystical tutor is nice with both of them. Such cards can get you what you need the most (fire/ice, chain of vapor, fow and disk) when you need it the most.
Time walk won't help much against an angry tog, a smokestack or a big mind twist running at you.
Ok, Timewalk is great and can make games.
But deck digging and efficient tempo in Landstill must definitely be considered. And that means sacrifing slots for that.
Note: did I mention I LOVE impulse?