Blue White deck, almost finished?

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Hey all, just wanting some advice on a deck I feel is nearly finished now. I've been refining it and it's starting to win fairly constantly now, just wondered what people's thoughts were on a couple of things.

First of all, here is the deck:

Land -

6 plains
6 islands
4 glacial fortress
4 azorius guildgate
2 seachrome coast


4 Drogskol captain
3 spectral rider
3 fog bank
3 mindshrieker
3 Geist of saint traft
3 latch seeker
2 angel of flight alabaster

Enchants and Artifacts -

4 oblivion ring
3 spirit mantle
3 mask of avacyn
2 negates
2 vapor snags
3 spectral flight
1 long forgotten gohei

So. I'm pretty happy with the creatures and the deck overall, obviously the point is to get the drogskols out and make everything hex and creature proof then attack.

My questions are: in regards to land, 1 plains and 1 island are being replaced with 2 more seachrome coast. Should I put more dual lands in though? Im thinking things like Sejiri refuge, hallowed fountain etc, or should I just stick with basic islands and plains?

Also, is there any cheaper but similar replacements for mask of avacyn? Apart from that I think my enchants are good to go.

Any help or suggestions or ANYTHING would be awesome. Thanks guys!

TL:DR - What would make my deck better?