are you racing? does your income or livelihood rely on the performance of your bicycle? if not, and if everything is working good for you, and you are happy, i don't see a need to upgrade. if you aren't happy with your shifting, maybe upgrade shifting parts. if you want better stopping power, maybe upgrade your brakes. upgrade the parts you want better performance out of. or, when something breaks, buy a better version of the one you had. if you really want to spend money on your bicycle, get new cables, then upgrade tires, then upgrade your drive train.