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The backstory of the Vizard as I understand it is that at one point they were normal Shinigami who had reached the limits of their powers, and began exploring the possibilities of using Hollow powers to complement their old ones. When Soul Society found this, they exiled them. If you go back and watch the conversation between Urahara and Ishiin after Ishiin kills the Grand Fisher, he talks a bit about this.
As far as fighting powers though, it didn't seem like Shinji used too much effort to beat around Grimmjow, and in the brief amount of time they use the power of their masks, their power levels bloom to a much greater level than the Espada's levels. As far as a released form of Espada being more powerful than a bankai-Vizard form, I really don't know because we have yet to see any of the other Vizard besides Ichigo even use their bankai.
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