Yep, I think that the 7s SIS mech's all have the the travel to span 8s, even if indexing (where used) going to the largest cog tends to be a good bit "off".
Getting the derailer's travel centered with the freewheel/cassette seems to be the bigger issue in most cases, so that the limits are more or less reached at both ends of the cassette so with all 8-9-10 cogs getting into the action if barely.
Even an older Stag's-Head or Exage Pre-SIS unit should span 8 cogs if well-centered. I generally find that removing 1-3mm of driveside axle spacing does the trick while still leaving 3-4mm clearance between the 1st cog and the dropout. This improves the chainline as well so is effort well spent imo.