Going from W6 -> W7 can be a noticeable increase in power, particularly because almost ANY W6 item can be imported into W7, including absolutely top-end gear like The Avenger, Muramasa Blade, and Ebony/Mantis armor. You start at level 5 with somewhat increased stats plus your import gear, but you don't have a lot of control over what those stats are. Which ending you choose in W6 has a significant impact on your faction relations in W7. If you come in with the T'Rang, for instance, it will take a lot of work and some good luck for you to have peaceful relations with the Umpani. It also affects your starting location, but that's only a major issue if you came with the Umpani. The T'Rang and Pen endings put you relatively close to where you'd start with a new game in W7, but the Umpani ending puts you halfway across the map in a pretty dangerous area, which can be quite a challenge when you're just starting out.
W7 -> W8 you're a lot less powerful than W6 -> W7. You still start at level 5, and you've got 9-10 extra stat points, but the gear you can import is really limited. There are a few normally unobtainable (or nearly unobtainable) items like Ymmu's Paw or the Mystery Ray that can be imported, and there are some other items like the Stun Rod that can make your life a lot easier. But you aren't anywhere near as powerful as you might think, due to the level scaling of the enemies and the mediocre nature of most of the gear you can import. Story-wise there's also essentially no difference whatsoever based on the W7 ending you choose. Choosing the T'Rang or Umpani will change your starting location but doesn't change anything else.
While it's a minor bonus for the most part unless you choose to import overpowered stuff from W6, I really enjoy importing parties. It gives the games more gravitas, I think.