Oh, thanks for the answers. So the DC isn't affected by other feats, how sad... And using shock trooper with brutal strike would nerf the DC anyways.
So knock down is unchanged? That means I can hit someone after I get the free trip succefull with improved trip? And I can cleave too? That would be awesome, and added to Robilar's Gambit this means some tricks. But I can't find the book that update it. This site discuss about it too, but I failed to see the knock down feat at the updates suggested
Knock-Down [Archive] - Giant in the Playground Forums
And what about (8)? That is mecanically weird, but it could make some conjurations spells more powerfull and illusions that cause damage.
I first thought about Blistering Spell because I like fire most. I know that, after the feat piercing cold, cold is the best element, but I like the flavor of fire spells. Don't you know some feat or class that duplicate the effect of piercing cold for other elements?