With synthetic oil 7k miles is not outside the normal operational life of the oil, in fact the original recommendation for oil changes when synthetic oils were introduced was 15k miles for many manufacturers. Some manufacturers have since reduced the recommended interval to change and there are many on these and other forums who change their oil even more frequently than 5k miles even thought they run synthetic oil.
Synthetic oil will get a dark brown color during normal use, it is not a very good indicator of the condition of your oil. Synthetic oil really does not break down like dino oil does, what actually wears out in the synthetic is not the oil itself but the additives that the manufacturer puts in like detergents, zinc compounds and cold weather additives.
I am glad for you that your car seems to be running better but I would venture that it is largely the placebo effect. My car seems to run better when I change the oil and when I wash and wax it. Don't know why but it just seems that way. If your oil was truly only 7k miles old then there should be no big difference in the way it runs.
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2001 Boxster S 3.6L, Zeintop
"Calling upon my years of experience, I froze at the controls." - Stirling Moss
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