I'm with the above. Fix/replace/upgrade what breaks, and ride as is otherwise, unless there's something that really bugs you.

In terms of bang for the buck, things like tires make much more difference than most components, though if the shifting is sloppy an RD upgrade might be someplace to spend.

When I got into bicycling wa-a-a-ay back when, the serious riders were of 2 camps. Those of who believed that it was bad karma to remove and replace perfectly good stuff, and upgraded bikes when stuff died. And those we called the Bike-of-the-Month Club, who always bought the newest, trendiest, hot bikes. IMO, once you fall into the "there's better out there", whether it's about bikes, electronics, photo equipment or wives, you're in for a long, unhappy, expensive ride.

BTW- you probably know this, but the only upgrades that really make a difference are to the engine, so focus your efforts there.