Hmmmm. After watching the guy playing the starlock in our game I'm somewhat puzzled about the whole notion that starlocks are underpowered.
1) There are PLENTY of powers keying off CON or CHA, especially with AP out. Focus on one of the two and don't worry about it. Sure you may be picking a few powers that don't have your pact rider, but frankly that isn't a huge big deal, plenty of their spells are perfectly adequate without the pact rider.
2) I'm just not seeing a massive difference in damage output. Its a bit lower than other strikers and nobody in our group has played a sorcerer so I don't have any comment on that specific comparison, but decent damage coupled with some fairly nice control effects seems nice enough.
3) Defenses... Hmmmm, well, this guy's AC is 4 points behind the rogue and 3 points behind the fighter and STR cleric, but one point AHEAD of the orbizard. Considering that he's concealed virtually 100% of the time his defenses are nothing to complain about. Aside from an occasional situation where he's gotten jumped by a monster the character has taken squat for damage. In fact it was a running joke in our group that the character would reach 30th level and never be bloodied. The other night a Gelatinous Cube came up behind the party and engulfed him. With his way high con this was only marginally threatening and he was never even close to bloodied, the cleric ran over hit the thing with a healing strike and he was never even close to seriously hurt.
Yeah, starlocks don't seem to excel at any one thing. They have mediocre defenses, modest damage output, modest single target control, etc. but the character isn't weak anywhere and always makes a major contribution in combat. His shtick is steady damage output round after round. The rogue and the fighter will roll out some really startling damage output some rounds, but they do almost diddly other rounds and then there are the times they can't get to melee with the target or its insubstantial or flying or etc. None of this bothers the 'lock, he just blasts away with EB or DR, and regularly drops in a really well placed tactical Hunger of Hadar which can easily turn a battle.
A feat for d8 damage would be OK, but I think its mostly a waste. Consider, he's already got an implement that jacks his curse damage to d8. He could burn a feat instead for that, but there are plenty of other feats he can really use instead to crank to-hit, lower saves, and all that good stuff. Not to mention throwing a slot or two into better armor!
So I can't say for the other pacts, but starlock IMHO is simply not gimped. Its a kind of funny class that you have to resist the natural temptation to split your prime stat on. But if you do build it reasonably well, the character is great, it just doesn't excel at ONE specific thing. I suspect the other pacts are similar. Its not easy to explain WHY the warlock is good, it just IS good. I think its that class you really don't want to have someone run right off the first day they play 4e though. The builds on them are a little odd and the guy playing mine had to rework the character a couple times at first to get it right. I know one thing, the party would sooner part with the rogue or the wizard than the warlock.