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1. You can use a bolt-on spacer (15mm should be okay), no real world issue. (Some Subaru guys might go "blah blah" about the wheel bearings, but if it's just for a test, it's not staying on very long)
2. The spacer will put the OEM wheel closer to the fender which has an effect, but then you're not considering the air under the car. The wheel is still the same width so it does not have the same frontal area a 9" wheel would.
3. Better have a damn good timing system. I'm willing to bet the difference will be so minute that recording errors will make the biggest difference in times.
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