I don't care how common rain is, Thunder should not be slashed on any of Latios' sets. Thunder should only be slashed on rain sweepers (Latios is a general sweeper) or Pokemon who intrinsically use Thunder like Jirachi.
The Life Orb set should be placed over the Choice Scarf set imo. The LO set is more common and more potent in OU. The Scarf set should really only be selectively used on teams that really need it.