Well they will probably be running 5 cell in open modified, with 3.5 (or variants of) brushless motors. There isn't a power loss there. I think what will happen is you end up with a bunch of guys playing 5 cell touring car ping pong on such a small track. Will be fun to watch, but will be a race of attrition.
I think we'll see that the 5 cell brushed cars in open mod are faster, much like on a larger track. Less weight, more corner speed, and less mass to accelerate. The cars are also more nimble with 5 cell, so they will be able to skirt around the tighter track easier than the 6 cell counter parts.
It's kind of lame that we're jumping on the "lets do everything that Europe does" bandwagon, but I guess that is the way RC racing in the USA is. 5 or 6 cell, I don't think it will hurt the race; it's still going to be fun to watch.
As for bringing new people in, it's just adding to the confusion... I don't think this is going to help, or hurt the hobby, it's just one more thing for guys like me to explain to new guys when they walk into the shop full of questions. There just aren't enough new people trying to come into the hobby..but that's a whole different thread.