There is one more option that i would suggest, modifying the der. I have filed then drilled, tapped and installed a machine screw into the cast stop on the back side of the upper knuckle that butts against the drop out tab. Much like the SunTour ders of then had. I used a M3 bolt (actually a Campy do adjuster and spring) for the angle adjusted bolt. The filing on the der stop was only to get a flat that is square to the adjuster bolt making it's drilling that much more easier.
I only did a few mods before I moved onto Asian units which shifted, IMO, far better. The only one I have left is the remains of the one that got caught up in the spokes of the wheel and was twisted and torn apart. here's a shot attempting to show the mods. You'll see the cast tab on th
knucklele has been filed and there's a remains of the threads still showing.
With this mod i was able to change the der angle and therefore the gap between the guide pulley and the cogs. The main reason i mention this (for the first time in 3+ decades) is that a der is easier to replace then a drop out... Andy