As I understand it, feats are specific to the base item, not to the item appearance. Having more bastard sword appearances should keep the feat bonuses for all added appearances assuming baseitems.2da is updated properly to look for them.
EDIT: OK, Mercurial blades have a natural +2 to multiplier. So why not make all bastard swords have that +2. As far as I see it, if you are going to add Mercurial blades, then there is little reason to have the generic bastard swords. And while your at it you can use the 3.5 variant for the katana to increase its threat range by one.