Hi Josh,
There's no hard and fast conversion chart that will say for sure. There are so many variables in this it's not even funny: What brand of kart is it? (I'm not asking, just pointing out there are 100s of different brands) How much does it weigh? What roller weights does it come with? What CDI does it come with? How much do you and your passengers weigh?
I could go on and on with questions (not looking for answers to those above right now) that would show that it's not as simple as saying "5mph" because it's a very individual result. A normal ballpark figure we throw out is generally 3-5mph when you drop down from 13g stock rollers to 9g.
You might gain back as much as 1/2 that with a no rev limit CDI - but maybe not. Since the rollers and CDI generally have nothing to do with one another in the engine other than the RPM level (lighter weights = higher rpms), the better CDI only comes into play if you've got a rev limiter in the stock CDI.
The performance CDI works best when you upgrade the air/fuel system since it allows better combustion of the additional air and fuel you're supplying (especially when paired with a high output coil and spark plug).
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- Mike
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