Should I buy a tuner??

I am thinking of purchasing the SCT SF3/X3 tuner from AmericanMuscle.com. I just have a few questions about the tuner before I pull the trigger on the purchase.

I have a 2000 Mustang GT (5 speed manual) that is my daily driver. I have the normal bolt on modifications (K&N CAI, Flowmaster catback) on my car as of right now. I plan to also purchase other bolt on modifications in the near future, such as an Accufab 70mm throttle body and Steeda underdrive pulleys. My questions to you, the forum are: How much performance will I gain with these few modifications with a custom tune? What should I expect as far as increased gas mileage with the tune? (I do not race my car so I would use the street and/or performance tune). And lastly, can the SF3/X3 tuner do things like read and clear check engine codes, adjust tire revolutions per mile, adjust idle, etc.?

If anyone has similar mods with a tuner your input would be greatly appreciated!!