I was looking for a thread or where it was saying where the spriest haste cap was and couldn't find it. I remember it being around 30% but don't remember the exact number. I'm sporting 4p t13 so that might change things and I know mastery is better than haste at that point but I'm still curious what the cap is.
There is no "cap". There are haste thresholds that provide a significant dps boost to hit, which after you hit will cause haste to lose value until you near the next threshold. More is always better, but at some points other stats are worth more point for point than haste until you can reach the next point. Check Kilee's guide on shadowpriest.com or howtopriest.com for a good explanation of it.