I'm not a great Sven player, but here is the way I see him:
1. Your stun is awesome...
but you're always low on mana.
Using your build this problem is kinda fixed by Urn and Magic Wand, but Soul Ring and Bottle can help you fix these problems, too. Maybe you should try them out.
2. When you activate your ultimate every hit of yours has the power of a nuke...
but you rarely hit people since they are concious of this and avoid it.
You maybe know these huge and incredibly strong monsters in movies who are sadly kinda slow and never hit anything till the agile protagonist finally slays them.
That's what you will be if you only focus on boosting your dps. But what if these one of these monsters suddenly appears right in your face so you can't dodge? That would actually make it dangerous, and that's why I recommend this handy item as core
Cool things you can do with it:
: You are actually in range to autoattack. In some games you deal more damage because of that than you would deal with Armlet. In some
: Stun, Ulti, Blink to the target and hit. It's pretty much unavoidable, you sit in someones face and if this someone isn't tanky he should be half dead already. With the help of a teammate it should be a secured kill.
3. Area of Influence
: The blink gives your stun basically 1800 range. You seem to be concious of the huge difference between 500 and 600 range, so I guess I don't need to explain how awesome 1800 range are. Helps a lot initiating ganks and even teamfights if your opponents tend to crowd together too much or just when you want to catch somebody out of position.
: You also get quite an unmanly escape mechanism, quite handy, really.
Btw: Your luxury choices are quite strange in my opinion, doesn't manta remove your ultimate?