Off the top of my head:
1) I’d STRONGLY suggest not to give your Castellan any magical suits of armour, only magic shields. It’s a shame to lose that 4+ armour save with 5+ Ward save vs fire. You pay for his Blackshard armour in his standsrd points cost, why not just improve it instead of removing it? I’d give him the Enchanted Shield and Dawnstone instead (or imo Stone Mantle and Enchanted Shield, adding the 5+ Ward save item if you feel like it).
2) I don’t have any experience with Hobgoblins, but I’d add those two blocks together and deploy them 5x8 to get lots of ranks.
3) Bull Centaurs run away fast. In my opinion they need either the +1LD banner or a Taur’uk with the Crown of Command in there.
All in all I would drop the Siege Giant and one unit of Bull Centaurs, and:
1) Add a Daemonsmith with the Dispel Scroll. Believe me, you’ll thank me when his re-roll ability causes your Deathshrieker rocket to hit bang on target. Lore of Fire for the occasional Fireball.
2) Add a K’Daai Destroyer. Enough said. It works…
3) Spend any leftover points on the last unit of Bull Centaurs. I’d prefer them to be 2x2 with the +1LD banner, great weapons, but I’m excessive with my Bull Centaurs!
I agree with Wifstrand on his points regarding the use of a Daemonsmith with Dispel Scroll and the K’Daai Destroyer. In addition, I recommend the following:
1. Dual Magma Cannons. Why? B.R.O.K.E.N.
2. Sorcerer-Prophet should ALWAYS be upgraded to level 4. This point is especially important if you are going with only one wizard. Talisman of Preservation is nice addition.
3. Depending on your strategy, Hobgoblin Khans are great for intercept/misdirect units.
4. You may want to consider fielding Hobgoblins with bows and shields.