I am curious if adding an airport express to a time capsule achieves the same results as adding an airport extreme
The answer is that it depends on your requirements.
The AirPort Extreme has a bit better antenna layout, so it will normally provide a bit better range than the AirPort Express. But, the performance of the two devices is very close, wireless wise.
If you need more Ethernet ports on the extending device, the Extreme has 4 Gigabit Ethernet ports, while the Express has 2 much slower 10/100 Mbps ports. And, the USB port on the Extreme can be used to connect a hard drive as well as a printer.
The USB port on the Express will only support a printer.
But, the Express has AirTunes capability while the Extreme does not.
If you only need to "extend" using wireless only, the Express is probably the better value, and its smaller size may make installation a bit easier.