I am not aware of an application that would do what you want, but another user may be able to help on that.
I'd like to know if the airport stations use 802.11ac or fall back to 802.11n.
Here, I am assuming that both AirPorts are 802.11ac devices. If they are not, then the devices will communicate at "n" level wireless.
If both are "ac" devices, since "ac" operates at 5 GHz, and 5 GHz is much weaker than 2.4 GHz, the AirPorts will need to literally have line-of-sight with each other if you want them to communicate at consistent "ac" speeds.
You can get a rough idea of the speed by moving your Mac laptop very close to the AirPort Extreme. Power off the AirPort Extreme for the "test" and log on to the wireless signal from the Time Capsule. This simulates the signal that the AirPort Extreme is receiving.
Hold down the option key on your Mac while you click on the AirPort icon at the top of the Mac's screen. Look for the Transmit Rate number and jot that down. Transmit Rate is the theoretical maximum speed on the network. The actual throughput speed will be about half of the Transmit Rate number.
Another way of saying the same thing is that you will lose at least half of the network speed if you try to "extend" the wireless network using wireless. An Ethernet connection between the AirPorts will maintain full bandwidth.
It may not be easy to run the Ethernet cable, but it is the only way to obtain maximum performance on your network.
