No I take exhaust because it's another slow to help in team fights, slow then while Abilities on CD and they are running, and to help me escape a gank. As I state in my Guide I take Deathfire gasp because I play Xerath with rarely going into Tripod because it telegraphs my moves so by getting closer for normal ranged abilities (Still not in there face) I can open with Deathfire then Magic chains for good damage and lower their MR, blast them with my first acrane barrage close the gap while they are stunned, since they will try to run after, hit with some lightening and then my last 2 barrages while they run and if need be I can tripod then to finish them off.

you take exhaust because you wont get in range for it? but why take deathfire grasp then?

Good Point. Like I said in the guide my core items is Sorcerer's Shoe Deathfire Gasp and Rabadons Deathcap. The other CDR Items Are situational like is you feel that you need more CDR to get to the 40% and your teammates won't let you get Blue buff or you feel Blue buff would be better on someone else for your team. I have used a builder and i wasn't sure if the builder was wrong with not letting me have more then 40% or if that was the cap. I will update my build to account for that. Thanks!

Hey, first off I really like the idea of lich bane as a situational item if the battle is more mobile and you have to move faster, as long as you don't abdicate that people should try to use the passive since being in auto attack range is usually bad news.
However i feel your stacking way to much cool-down reduction, there's few people better to take blue buff then your AP carry but with your build your already over the hard cap of %40
(%35 from items %9 from masteries) so blue buff is just mana regain for you.
if you took just the death fire grasp you'd be at %24 and hit the cap with blue buff letting you build something that gets a little more bang for your buck.