You need DIS, as well as several other things. Last session I was using DM Battlescreen to track combat. At the end I wound up with my archer Ranger doing 37% of all the damage in a 7 player party (42% would have been 3 players worth of damage). This forced me to seriously look at all the PC's in my party to find out why there was such a HUGE variation in damage output, and I found many things that explain why my combats have been taking 3-4 hours at n and n+1 difficulty. And let me clear up that it's NOT because the Ranger was doing too much damage, but the other strikers were not doing enough for various reasons.
The original point stands though, DIS requires a magical implement in the off hand. It gives a damage bonus equal to the enhancement bonus of the off hand implement, which in your case is 0. It may well be an unforeseen side effect of the inherent bonus system, but it exists. You could easily house rule it and in CB just give yourself a magic implement in that hand to make the numbers come out right. Inherent bonuses are an optional rule, they may not interact perfectly with every other part of the system.
I agree, optimizing strikers to increase their damage output can have a really significant impact, especially at mid-high paragon. Even so I would suspect that the other strikers you're comparing to are a bit sub-optimal. Another possibility is those players simply aren't doing a very good job. Are they coordinating their attacks with leaders to get to-hit bonuses? Do they exploit flanking? Do they take advantage of things like Prime Shot? A lot of the variation I've found comes from how the character is played. A good player should be able to do pretty well even with a striker with no special feats.
Heck, the other day I took a look at the character sheet for the level 10 starlock in the group I'm DMing for. This guy has NO feats that do anything for his damage output. The one feat he has that helps his DPR at all is Warrior of the Wild (gives him HQ once per encounter). He's still doing really excellent damage, and with a class that by reputation generally has a lousy damage output. He's just good at tactics. He's always taking advantage of leader bonuses etc. He's done a good job with his ability scores, picks good powers, uses them effectively, and just generally makes sure he's eeking every last drop out of his character. I guess he just really has better uses for his feats. I'm sure he could pump out a lot more damage if he'd take Expertise and DIS and Weapon Focus, but good play is at least as important as tweaking your build when it comes right down to it.
By comparison the rogue in the same party has all sorts of feats, Weapon Expertise, Weapon Focus, Backstabber, TWF, etc. That particular player honestly has just never quite grokked the 4e rogue. Her damage output is less than the fighter's most of the time. Lately she's been getting a little smarter, but I'm pretty sure in most fights she's done poor damage. The build of the character is fine. It may not be tweaked to the Nth degree but its a totally solid build. Its all in the play. The warlock stomps her on total damage output encounter after encounter.
