Given that throughout the 70's, 80's and most of the 90's most tandems were fitted with cantilever brakes, and from the 90's on cantilevers (to include the linear-pull variety) continue to be the default as the OEM brakes on the vast majority of tandems sold, I think one could safely say that yes, using some type of cantilever brake on a tandem that will not be used on challenging terrain will provide stopping power that is on par with the vast majority of well-made tandems in use today.

Are the Avid 7's the best option? Highly subjective and best addressed by teams who are already using them. I have Arch Rivals on my Ventana Marble Peak (solo bike) with the Single Digit MTB levels: very nice, almost as nice as the also discontinued Arch Rival Supremes which were also an excellent linear-pull cantilever brake for tandems, or so those who used them said.

Is changing out the disc brakes reasonable? Again, highly subjective and best addressed by looking in the mirror and asking yourself: "Will this increase the satisfaction and enjoyment I derive from my tandem more than anything else I could do with the same expendature of discretionary income and without any undue change in the level of risk I must already contend with as a cyclist?"

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FWIW, this is our tandem brake ownership history as a 280lbs team. Our local on-road terrain is moderately hilly with local mountains that feature many, long 4% - 8% climbs/descents, and a few that hit between 12% and 20% but those tend to be short. Will will occasionally venture off to other locales where the rare, longer, steeper descents 'could' warrant supplemental hub-mounted brakes.

• '95/96 Santana Arriva: XT cantilevers with XTR shoes and pads. Seemed like they worked just fine.
  
• '98 Erickson: Campy Record differential calipers front & rear with supplemental Hope mechanical rear disc. The calipers seemed to be superior to the cantilevers and the Hope disc, although only installed and used a few times, did exactly what it was intended to do... create drag to slow descent speed on very long, steep and challenging switchbacks. No how, no way would that rear disc stop a tandem by itself even on a flat and level road.
  
• '98 Cannondale MT3000: Shimano XT linear pull / V-brakes. Fine for most single track, but we melted the minimal pads off on a fast fire-road descent and, well, that was the end of that for V-brakes on our off-road tandems.
  
• '00 Ventana ECdM: Hope 04DH 4-pot open hydraulics. Simply awesome stopping power and very high heat capacity for off-road use. However, we still succeeded in achieving heat-induced disc lock on two occasions.
  
• '02 Erickson: Campy Chorus differential calipers front & rear at first with Hope mech. rear disc / drag brake. Same as '98 Erickson: excellent rim brakes for our riding style and weight. Rear triangle modified to allow Hope disc change out for Avid BB Road in '04 as primary/sole rear brake matched with Campy Chorus differential front brake: best yet. 6,000 trouble-free miles with the rear Avid.
  
• '02 Ventana ECdM: Hope Enduro 4-pot open hydraulics. Same awesome stopping power as previous brakes in a smaller package and very high heat capacity for off-road use. However, these suckers have always squeeled like a stuck pig despite a variety of attempted fixes. Just haven't bothered to try other brakes and, well, it lets riders ahead know that we're behind them.
  
• '07 road tandem: Campy Record differential caliper front and either caliper or Avid BB Road in rear depending on how I feel when I built it up.

Many other brake and brake systems have been test ridden, worked-on, or researched over the years but these are the ones that we have selected for our own use and lived with. Guess what, these and all of the other brakes work "pretty well" when set-up per the manufacturers specifications, properly maintained, and used within their design limits. Rider experience, judgement, and technique seem to have a greater impact on brake performance than anything else, short of a screwed up brake installation. Some brake systems -- from certain cantilevers to discs -- are complex enough to screw up without trying hard.