After playing Sejuani in the PBE, she's actually doing surprisingly well compared to most people's expectations, including my own.
I definitely think a small buff to the rank 1 of Northern Winds could be used though. Otherwise, she doesn't feel bad at all. The increased XP gain helps immensely as it lets her hit 4 faster where she starts doing actual damage.
If anyone else wants to try it on the PBE, I used 0/21/9, AS marks, Armor seals, MR glyphs, Armor quints, and starting with Machete and 5 potions. Want to try 0/9/21 next with a few Armor glyphs to offset some of the loss of Hardiness to get the increased XP, biscuit, MS, and extra starting pot.