The warlock is not overpowered at all. Look at it this way: at 10th level, he can use one eldritch blast per round, doing 5d6 damage. In comparison, a 10th level sorcerer with Cha 12 can cast up to 20 scorching rays per day, doing 2x(4d6) damage per casting. Sure, if the campaign is such that there will be a need for more than 20 attacks per day, the warlock may have a slight edge. But then, you're not taking into account all the other stuff the sorcerer can do (and bear in mind that sorcerers are generally underpowered).
The 10th level sorcerer could instead throw a bunch of fireballs, doing 10d6 damage to all enemies in a 20 ft. burst, which means that he does as much damage as a warlock even to enemies who pass their saves. A warlock who misses his ranged touch (and it has happened to me a couple of times) does no damage whatsoever.
At 10th level, the sorcerer knows 15 different spells of levels 1-5. The warlock of same level knows only 6 invocations. Additionally, that same warlock probably spent at least one (but more likely 2 or even 3) of his invocations on eldritch shape - so his flexibility is reduced even further.
As for Maximize and Empower SLA, they are nice, but they only bump the warlock to be on par with equivalent-level characters. Look at this: a 10th level warlock can empower and maximize his eldritch blast 3/day, doing 30 + (5d6/2) damage, average 38 damage, per round. 38 damage per round? That's less than what a moderately optimized fighter can do at that level, and WAY below what a min/maxed fighter can achieve. But the warlock really cannot be min/maxed much beyond taking those Metamagic SLA feats. If I live to see 13th level, I'll take a level of fighter to get Improved Critical for my eldritch blast, but that's about it. I could take it at 12th level, but that's reserved for Quicken SLA, of course.
EDIT: needless to say, unlike a fighter, a warlock is REALLY screwed if he gets grappled, since invocations use somatic components. He's even weaker than a sorcerer of wizard in this respect, because his version of dimension door (flee the scene) is also an invocation and also requires somatic components.
About the <3... hey, it was 2 am and I was tired, using an emoticon seemed like a good idea at the time.