I recently got back into Ubers so I think I shall rate.
Your using a Specs set and spread with leftovers. Specs is just as bulky and only lacks leftovers recovery which it makes up for when you almost 2hko a Ferrothorn with a 4x resistant Water Spout. So personally I would you Specs.
Though Dialga is rather defensively bulky it is a better idea to invest in Sdef. This allows it to take hits from Pokémon like Palkia and Kyogre better. Its Def is already pretty good and Giratina-A is actually bulkier thanks to its massive Hp stat. Dialga is a great special wall thanks to typing. Pair that with a defensive Giratina and you have a nice little core.
So as much as I hate suggesting standard sets, I'm suggesting standard Sdefensive set.
Pressure | Modest | 252 HP | 56 SAtk | 200 SDef
Draco Meteor | Thunder/Fire Blast | Stealth Rock | Roar
This guy is actually a bit of a monster in Ubers boasting 2 120 base offensive stats with Download and a brilliant movepool to compliment its Atk. Though you are going for the exact same setup as everyone else I have seen. U-turn is possibly the best move in the game. Genesect has stab and a possible download boost to use U-turn very effectively. So why give it Hasty and no Atk investment then? A max attack Neutral natured Genesect hits 339 Atk. With a Download boost it can deal 50% to Specogre. Its a new tier and you nee to adapt the sets so they fit the tier. Giga Drain and Ice Beam even with the SAtk boost will not be hitting any of the tier hard excluding a few. Try this set, I find it works wonders and not only boasts power but gives you the upper hand in momentum.
Genesect @choice scarf
Download | Naïve | 248 Atk | 4 Def | 32 SAtk | 224 Speed
U-turn | Explosion | Ice Beam | Iron Head
Anyway the evs lets it out speed scarfed base 90s and 95s meaning your still out speeding everything it would with max speed apart from opposing Genesects in what would have been a speed tie. U-turn ohkos Kyogre with a crit which is nice. Iron Head is a nice stab that should deal nice damage to anything not resisting it after boost. It also nails the oh so common Kyurem-W. Ice Beam is primarily for Rayquaza and the 32 Satk investment means even without the boost it will always ko Rayquaza. Explosion with a download boost is almost unstoppable. It may seem inconvenient and you will not find yourself using it so often but it takes out any major threats like lets say Arceus. Though U-turn will be your main move being able to damage the foe very nicely while keeping momentum.
A physically defensive set benefits your team better with the changes being made so just switch round the evs and use Impish. Apart from that its great.
I hate this thing in Ubers. I see it in Ubers because its a tad unbalanced in Ou. It has no real niche to it. It lacks bulk and its stab Thunder will be doing not much more than Kyogre Thunder if not less. Personally I prefer its brother, Thundurus-T as it has decent speed but a much better Satk and Volt Absorb. It doesn't synergise with the team well to add.
Try Mewtwo it hits 1000x harder
Mewtwo @choice specs/Scarf
Pressure | Timid | 4 Def / 252 SAtk / 252 Speed
Psystrike | Aura Sphere | Fire Blast/Thunder | Shadow Ball
Sorry the finished answer did not come sooner.