Ask the shop to define "worn".
If the upper strut mounts are shot, they're inexpensive, just replace them.
I've had plenty of Koni struts on many different cars over the years, and <15,000 miles would be a disastrously short lifetime, unless the install was pooched somehow.
So, looking at your sig, I see H&R springs. Did the install include proper bump stops on the strut piston rods and boots over the strut rods? Koni does have a warranty, but if they see damage to the piston foot valve from not running proper bump stops, they'll void the warranty.
Are the struts leaking oil at the piston rod seal?
Are the piston rods bright and shiny and free of pitting and corrosion?
Struts can fail in several ways:
1. They can lose their ability to dampen the spring, and results in a bouncy rebound. This is easy to test by pushing down hard on the rear bumper and watching how the car rebounds. If it rebounds cleanly without overshoot, they're probably fine, but if it rebounds and oscillates, they've lost their damping ability.
2. They can lose their gas charge, and Koni's will do this, sometimes in only a few tens of thousands of miles, but this generally won't affect their performance or damping, and the only way to test this is by removing the strut assembly from the car, removing the spring and testing if the piston rod rebounds after being compressed into the strut cartridge. No slow rebound means no gas, but it's not something you would notice driving the car around on the public roads.
3. Really old struts can suffer upper bushing wear where the piston rod enters the strut cartridge, this can cause noises and inability to hold a suspension alignment. Again, the only way to test this is by removing the strut assembly from the car, removing the spring and checking for excessive (axial) piston rod play.
You can do number 1, ask the shop if they removed and disassembled the strut/spring assembly and checked for gas rebound, piston rod axial play, and clean, corrosion-free piston rods.
Creaking could simply be a dry strut piston rod, this can be alleviated by disassembling the strut and wiping some oil on the piston rods.
The rear suspensions on these cars are prone to clunks and bumps when the bushings for the anti-roll bar end links rot out where the links pass through the rear control arms. In most cases, you can't see how bad the end link bushings are until you take them apart.
Additionally, in locales where this is a lot of winter road salt used, the anti-roll bar mounts on the rear subframe can rust away or fracture, leaving the ARB poorly supported, which makes noises and clunks.