What do you guys think we need to cut for a black splash with 4 Thoughtseize main?

I am playing this deck since GP Ghent and I'm very, very happy with it so far, since I'm getting quite good results with it. Went 1st, 1st, 3rd, 3rd at my last 4 local tournaments, for example.

Currently playing this list:

2 Island
3 Volcanic Island
3 Misty Rainforest
3 Ancient Tomb
4 Scalding Tarn
1 Mountain
2 City of Traitors
1 Tropical Island
1 Polluted Delta

3 Show and Tell
4 Burning Wish
3 Emrakul, the Aeons Torn
2 Griselbrand
3 Daze
1 Spell Pierce
4 Force of Will
4 Brainstorm
3 Preordain
4 Omniscience
4 Ponder
1 Overmaster
2 Flusterstorm
2 Intuition

Sideboard:
2 Red Elemental Blast
3 Defense Grid
4 Leyline of Sanctity
1 Time of Need
1 Grapeshot
1 Show and Tell
1 Petals of Insight
1 Pyroclasm
1 Overmaster

Right now I really want to try out a black splash for 4 Thouhtseize main, because it really sucks losing to 1-Off Oblivion Rings out of my opponents sideboard. Happened twice the last 2 tournaments and were my only losses.

I think cutting a Mountain and a Tropical Island for 2 Underground Sea is an easy choice. As well as cutting a Defense Grid in my sideboard for Eye of Nowhere, to get an additional out for O-Ring.

I am not so sure what to cut for 4 Thoughtseize, though. The lonely mainboard Overmaster gets a cut, that's for sure. Aside from that, I really don't know what to cut. Don't want to cut any cantrips, since we really need them for our 3-card-combo. Pretty much leaves the choices to either Daze or Flusterstorm/Spell Pierce. Right now, I guess I'm taking out 3 Daze.

What do you guys think?