Let's see, what can I do at this level with just your higher level slots using an Alpha Evoker 10. All cases assume that you hit or succeed.
Cone of Cold: 60 ft cone of 10d6+2 damage with DC 15+Mod 8-62 Damage Avg. 37
(Not as good an option as this next one.)
Empowered Fireball: 20 Foot radius burst Long Range spell for 10d6+2x1.5 DC 13+Mod 12-93 points of damage Avg. 55.5
(This is a considerable boost in the damage. With one spell slot two levels higher, I just increased my damage by 18.5 on average. That's a great amount of gain but still not going to bring down the Giants by itself.)
Maximized Scorching Ray: Close Range Ray spell for 2 of them Damage 4d6+4d6+2 for 50 points of damage. Avg. 50 at one or 24/26 to both
(This is not as good as the empowered Fireball of the same level of spell slot. Plus I need to make two attack rolls and have two opponents. Not really going to do much overall.)
Quickened Burning Hands: Extremely close range 5d4+2 Damage spell for 7-22 points of damage with save DC 11+Mod. Avg. 14.5
(Not really much to say about this. It's just easier to use a weapon.)
So from all this, Empower Spell is the most effect metamagic feat for a Evoker. But is this the most effective method of taking out both monsters? I mean, you use 2-9 spell slots to kill these monsters with empowered fireballs or you could use 2 Slots to use Hold Monster on both of them. Oh wait, that spell has a save each round so that in 10 rounds it has 10 chances to make the DC 15+Mod with their +7 will save. SO a total of 3-? rounds are expended just coup de graceing the creatures whom are making saves every round. If any of them break free from the enchantment, then you would have to expend more resources.
IF you just did average damage with the empowered fireball, you just took out those same giants in three rounds. That's kind of comparable.
Lets do another example. Cloudkill. The damage that the Giants are taking is 1-4 Con a round or avg. 2.5 Con while they are in the cloud so a well placed cloud could effectively take the Giants two rounds to get through. That's only 2-8 Con damage or 1-4 Hp/HD(In this case, 14-56 points of damage) Avg. 5 Con damage or 3 HP/HD or a total of 42 points of damage to these monsters. That's less effective than the empowered fireball. Now I'm not saying that it's not possible to keep the monsters in the cloud but the spell by itself is less effective. Lets look and see if you could get the spell to function to it's maximium effect. I would have to do 5-19 rounds of con damage to take out the monsters. That's powerful but I would still prefer 3 spell slots than deal with the monsters for 5-19 rounds.
Ok so the Fireball is looking like it should. Hold Monster is the champ here but it's drawbacks make it as difficult to guarantee success as any of these other spells.
In my examples above, I just picked out some of the more powerful spells from the SRD, and what did I learn, an empowered Fireball is deadlier than a higher level Evocation spell at 10th level but everything is still in balance because in two more levels, that spell begins to balance out against it's counterparts like cone of cold.
Now my test is not complete and would like some imput without flaming. Did I pick the right spells to compare the damage to? Are these the spells that are supposed to be better than the damage spells of their level? Please let me know so I could put them to the test.