10k Golden company vs 15k Stormlanders

[Lord Lannister]

I don't think you're going to see anything resembling a united Stormlands force resisting the Golden Company. Sure the actual men may be there, but the leadership is not. Renly is dead, Storm's End is still held by Stannis, but is under siege and Stannis himself is on the other side of the continent, presumably with at least some of the strength of the Stormlands.

That leaves each of the individual Baratheon bannermen on their own with whatever remnants they may have in the aftermath of the Renly/Stannis conflict and the Blackwater. While I suppose it's possible some of them may send a token force to participate in the Tyrell siege of Storm's End, to hedge their bets remaining loyal to Tommen, most are going to sit on their hands and wait and see what happens. There's really nothing to gain by jumping in and so much to lose.

The real question will be if the token force under Mathis Rowan fights the Golden Company, retreats at their approach or dips their banners. Likewise with Stannis' castellan, Gilbert Farring if he decides to hold fast against the Company as he did the Tyrells. If Storm's End falls to the Golden Company, you likely will see the Baratheon bannermen considering their future. After all there is a prominent lord from the Stormlands who could be named Lord Paramount by a new king with that army.