I would only BSC 3db, not 4-6db. That'll leave you with almost 93db sensitivity. And since the filter is parallel and has a resistive element in it, you're not really degrading the efficiency of the driver. (efficiency and sensitivity are different)
There is a good chance 8 watt could still get you to 100db, and 20w would get you to 103db. So unless you're entertaining a party, I don't foresee serious problems.
Usually more power is good, but for SET I don't necessarily think that, I think whatever SET sound best is best, as long as it isn't a total flop in power on the wrong speakers. Tubes sound louder, too. They clip very nicely, so 8w often is a bit more.