I don't typically blow an entire wad in a single round. I like to save Low Slash for when I miss with my standard action, so I can still get my sneak attack damage in for the round.
Always a good idea - something the rogue in our home group has picked up on as well.
At 7th level, I was usually doing 1d4+5+2d8+3, for about 20 points of damage in a typical round. Occasional crappy dice notwithstanding, I was at +16 to hit with CA (which I almost always had), so i was hitting most of the time. Still, 20 damage per round doesn't seem particularly high to me for a 7th level striker.
Actually, I don't think it's that bad. A big part might be the weapon, which I imagine is an obvious observation.

If you were using a rapier instead, that's two points per [W] more. (( but then it cannot be thrown and takes a feat and so on, i know *shrug* ))
My archer, at 7th with twin strike, did 2d12 + d6 + 10 iff he hit with both shots, but his attack was only +13 (and rarely getting CA) so 60% of the time or so, it was d12 + d6 + 5 per round. And no one batted an eye at consistent mid-teen dmg every round, with good rounds in mid 20's.
At 8th level I'll be doing 1d4+7+2d8+5, for typical damage of 24. So, in your scenario, if I blow Low Slash and an Action Point in the same round and I have CA on the enemy, and I hit with all three attacks, I rate to do (1d4+7+2d8+5) x2, plus another 1d4+7+5 for low slash, which is an average of 62 points.
Again, I think that's pretty nice for nothing but two at-wills and the minor encounter power.
Throw in a [2W] encounter power that adds your strength to the dmg regardless of SA (or some other boost, such as Bloodbath) and you can have one very nice nova round every fight. And *that's* what a striker should be able to do, imo.
Also, regarding Slaying Action, we usually have non-combat encounters between combats, so with most combats we earn a Milestone and an additional Action Point.
Yeah, my expectation when considering feats is to assume an AP in any fight you'll want it. Either because you forgo using one in one fight or because you get those other scenes mixed in there.
So slaying action, to me, equals 2d8+5 dmg per fight. I'd call that worth the slot.
Whereas other feats of a similar nature (AP or racial-power dependent) often look soft because their once-per-encounter restriction is Not paid off with a decent-enough benefit.
Cheers.
-VIC