I wouldn't use wider than a 7" wheel.
I've seen people run 8" wheels, and I've seen successful 7"wheel\8"tire combinations before. I think the 8" wheel requires modifications for fit and rubbing issues, though.
7" wheels are the most you will realistically "use" on the street. Too wide WILL slow you down. More than 7" on the street (with this chassis at least) is impractical, if you use a larger wheel, put track tires on and save them for track days.
In my experience, 15x7 and 16x7 are the two best sizes for EF street applications.