I have to agree with Jibaku, Ice Beam is a must on choiced sets. Its essentially the same thing as running Draco Meteor + Dragon Pulse on the other choiced dragons except Ice Beam is stronger than Dragon Pulse, hits a majority of steels for neutral damage(!!!), and has that random freeze chance at the cost of losing damage on a few pokes. Most of the pokes Ice Beam does less damage than Dragon Pulse on are only coming in as revenge killers anyway, Palkia takes 76% minimum from Timid Specs Ice Beam, more than enough to keep Palkia from directly switching into you.
Now for something completely different: Modest versus Timid
I'll start by saying the Scarf set needs to be Timid to outspeed the scarfed base 90s that's a given.
For non-scarfed sets, Modest gives you more power while abusing the fact that most base 90 pokemon don't run max speed without a scarf, Reshiram being the only poke that seems to do so. Timid obviously lets you outspeed/speed tie other Kyurem and unboosted Rayquaza, but I don't think that outweighs the extra power from Modest. Timid 170 only gives 1 more SpAtk than Modest 150(Dialga/Reshiram) before and after applying Choice Specs. Essentially, you're trading Bulk, a SR resistance, and a better move pool for Speed, a better ability, and an SR Weakness in the case of Dialga. In the case of Reshiram, you're trading Fire STAB for Speed and better SE coverage. You can still outspeed everything important as Modest, but you also gain a bit of bulk and do noticeably more damage than Modest 150s.
<Witty comment here>