The biggest benefit of a cage or roll bar (Occupant protection aside) is additional chassis stiffness. A flexing chassis can affect suspension geometry, alignment, steering response and ultimately...traction. This is of course offset by the additional weight of which a fair amount is located above the carís center of gravity.
As for wheels in the stock class, it's my understanding that they need to be the same width and diameter as offered by the manufacturer on your particular model, but the offset can vary up to 1/4 inch in either direction (Negative or Positive).
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