I would say that Expert Belt is perfectly viable, but White Herb is the item of choice to prolong sweeping, incase the opponent carries multiple priority users or a mon that can actually take one hit (think bulky support Arceus). Now, the power drop isn't much noticeable, but for example, Lugia will not be able to take an E-belt boosted thunderbolt after Stealth Rock. I am in the belief that rain is the best weather for SmashPass because you will be able to use Thunder instead of Thunderbolt, and some Lugia are EVd in a way to survive a +2 Thunderbolt from Dialga. Being able to get past specially defensive Kyogre (which of course summons rain, but then again, having Thunder and relying on opposing Kyogre to summon rain is a bad idea) is also nice. The best moveset is in my opinion Dragon Pulse, Aura Sphere, Thunder and Brick Break (nabs a 2HKO on Chansey) if you are running a Kyogre on your team. But then again, if you are running Groudon, Flamethrower is a nice option, as well as Expert Belt.
Lugia has great bulk and can with support wall a significant portion of the metagame. The threat of Substitute+Toxic Lugia is always something a team must be prepared for, as it can actually very easily sweep- as strange as it might sound- should you lack a steel type or a rapid spinner. Being spikes-fodder is of course bad, but being forced out my rapid spinners when behind a substitute is even worse, and that sadly makes it hard for Lugia to work very well. However, under circumstances where you manage to take out the opponents spinner and/or steel-type, Lugia can be very, very annoying to take down. Another thing that have made the Lugia a little bit sadder was BW2 addition of Kyurem-W, who can simply smash through Multiscale with a STAB Ice Beam. If you can cover Lugia's flaws with correct team support, it can still work, though.
