For lens work, I would recommend 10nm or greater. So go with the 13nm bandpass. While you'll still experience some shift, though it will not be severe. In fact, with a 13nm filter you could image at F/2.8 if desired.
This is a good read -
http://www.hbastro.com/Telescopes/Fa...derations.html
While you don't have an astrograph operating at 8" - F/2.6, the concept with Ha filters remains the same. Take note of the 12nm Ha BPF graph detailing the shift in the URL (also posted below).
This aligns with what I mentioned in the other post - "...the shift results in a slight drop in transmission efficiency i.e. 90% to 75%, thus on a worst case scenario, you would need to increase your exposure time...however with fast optics, this would not be a huge disadvantage." Ultimately, the interference filter would still be operating fine, so you'll get the contrasty Ha emission line, its just that the transmission efficiency may not be as advertised. Having a larger bandpass addresses this issue.
Hope that helps.