I have a set of 4th gen SLP Firehawk wheels. I'm trying to decide whether to use them or to go with a set of Z06 wheels

First, they weigh 22.8lbs each. Are the reproduction Z06 wheels significantly heavier?

Second, I see these wheels described as 9" wide and 9 1/2" wide. Measuring from the inside of the mounting lips, which I understand to be the correct method for determining wheel width, I get 9 1'8" Am I doing this correctly?

Third, laying a straight-edge across the backside of the wheel and measuring to the mounting surface, I get a backspace of 6 7/8". However, measuring backspace in this manner uses the OUTSIDE of the mounting lip, not the inside. If I want to establish the correct wheel centerline and offset, I think I'm going to have to use the measurement between the outside of the mounting lips (9 7/8"), not the inside. Is this correct?

Using the outside-to-outside width measurement, I get a centerline of 4 15/16". a backspace of 6 7'8" minus the centerline of 4 15/16" leaves an offset of 1 15/16", or 49.21mm. Is this correct?

 

Yup, looking at this: http://www.rsracing.com/tech-wheel.html#backspace

you do establish the wheel centerline by measuring from the outside of the wheel lip to the outside of the opposing wheel lip