We have gone on long(ish) tours on three tandems. My take is that any pannier system that is loaded up will cause instability when the captain wants to stand out of the saddle. This includes a new CoMo frame without the long diagonal tube and two other tandems with the diagonal tube. to the best of my knowledge these are moderate trail frames and the CoMo is probably considered a high trail frame, which I do not like now that I understand what that means. All tandems with moderate rear pannier loads will instantly wobble as soon as you stand up. I do not know if this will be the case with loaded front panniers. It's just a mass and leverage problem.
Our solution is the BOB trailer. much easier to handle. no wobble. carries as much as 4 paniers.
I have never noticed any performance issues between our modern CoMo w/o the diagonal vs a previous custom TiCycles with the diagonal. Both have the same problems when loaded, both ride fairly equal when unloaded.