I have a 2011 Mazda 3 i-Sport with a manual transmission and a 2.0 L 4cyl engine. Should I upgrade with aftermarket parts?

It depends on the how good is your engine and transmission. make sure the transmission is in a good condition to handle that kind of power. I recommend Racing Spark Plugs. I have tried them before. Good Luck

Unless you are willing to invest the research, time and money into a turbo system, you will not likely be too happy with the results. I own an N/A 2.3L HB that's been modified since about 2005. I was very patient and did a lot of careful shopping for the best prices, read all the customer reviews and comments, etc, and the results are still less than stellar. No change in mileage. Slight loss in low-end torque, slight increase in upper rpm power. To me, probably the biggest waste of money was the intake. I love that it helps clean up the engine compartment, but it does absolutely nothing for power, and even at the deal I got was $200 (new/used from a forum member who had two.) I installed the Vibrant exhaust ($640 back in 2005,) and I love the way it sounds. I like being able to hear the car, when I should shift, etc, but in the end you do lose a little low-end torque from the bigger tubing. Tuners are useless as well. $500 for MAY-BE an increase of 7 hp? The 2.0L is well suited to a turbo, but will cost you $3-$5k. And there are the reliability issues after. Every time you increase power at the engine, you are also adding strain to the rest of the vehicle, specifically the drivetrain in this case. Just something to keep in mind.