Welcome Aboard!
I see you are flying the German flag. I spent a lot of time there, beautiful, it really is. Nothing would be nicer than cruising southern Germany. I have been catching the Chrismas markets for years. Missed this year though; healing from a new knee in November.
In any event, you asked about the Vance and Hines Oval Slip-ons. I put them on my 2003 SERK after a lot of consideration, and can only say I am very happy with them. According to Vance and Hines, it is their highest flowing slip-on, which we need with these bikes. The look and the sound is right on. Those big Ovals sticking out from under the bags look great.
However, given the free flow of these mufflers, you really need to richen up the fuel curve. And given that you have to do that, I would open up the input breathing by putting on the Screaming Eagle air cleaner (I put on the Arlen Ness Big Sucker Stage 1 kit, its a much better design). Then finish the job with an ECM re-flash using the Screaming Eagle Race Tuner. Harley offers a great program specifically for our motors with the exhaust and intake improvements in the Race Tuner kit. At this point, you have done about as much as you can do (of course you can dyno tune) without getting into the motor.
This is the stage I am at right now, and I can say that the performance improvements have been substantial. There is a real seat of the pants difference. I also lowered the overall idle speed package in the advanced mode, and got much more of that great Harley sound. It just doesn't sound right idleing at 1000 RPM.
I may get into the motor (still nervous about Twin Cam problems, read American Iron Magazines January issue, article on page 32). There still appears to be cam, chain, and follower problems. If I do go to gear driven cams, I will obviously upgrade and do higher compression forged pistons at the same time. But for now, I will continue to fine tune and optimize my existing package. Its a blast. Soon you SEEGs will be able to experience the fun us SERK guys already take for granted. Believe me, it will be worth the wait.