OK well if your problem affects internal traffic (streaming AirPlay) and external traffic (website access) then that points the finger at your Time Capsule. The fact that you've only noticed this since you connected your new iMac might be a red herring but may also be a hint at the cause. I'd start with pinging the time capsule and see if you have periods where you can't ping it. So open Network Utility > Ping and enter the IP address of the time capsule (probably 192.168.1.1) and set it to ping an unlimited amount of times, you'll see output like that below:
Ping has started…
PING 192.168.1.1 (192.168.1.1): 56 data bytes
64 bytes from 192.168.1.1: icmp_seq=0 ttl=64 time=1.197 ms
64 bytes from 192.168.1.1: icmp_seq=1 ttl=64 time=1.158 ms
64 bytes from 192.168.1.1: icmp_seq=2 ttl=64 time=0.996 ms
64 bytes from 192.168.1.1: icmp_seq=3 ttl=64 time=0.970 ms
64 bytes from 192.168.1.1: icmp_seq=4 ttl=64 time=1.103 ms
This, in case you don't know, is just sending a packet to the IP address of your time capsule and the time=xxxx on the right is a measure of how long it took to get a response. Now, if something is happening to slow down the network response time, you'll see it here. As you can see there is very small fluctuations (the units are milliseconds) in the response time but you'll be looking for larger differences, such as 1000ms or even dropped packets. Obviously this is only useful if you're seeing the issue every few minutes rather than once a day. Once you prove that you have captured an issue, try the same thing while plugged directly into the time capsule by cable. If you get similar symptoms then I'd point the finger at the router. See how you get on with that and I'll update the answer when you come back if I can suggest some other steps. Good luck!
While quite possibly you do end up with a lower battery life in your tests than the one advertised, you need to take into account the following:
Test conditions
Many factors will determine the outcome when testing the battery life, like signal strength, iPhone settings, screen brightness, the actual websites being loaded, background processes and much more. FYI, apple discloses it's test conditions, you can read about them here. If you truly want to compare, you should as best as possible mimic the test conditions Apple used.
Test accuracy
While I have no problem taking you for your word your battery drops 1 percent exactly every 5 minutes while browsing, that doesn't necessarily mean you end up with 8 hours and 20 minutes of battery life. There is quite a large margin for error when you only sample such a small period, and the only real way of checking Apple's claims would be by actually trying to surf the web for 10 consecutive hours.
Concluding, while Apple's test conditions are semi-optimal for their testing results, they have a better reputation for test accuracy than some other manufacturers. They do test the iPhone with almost factory default settings, but specifically for browsing the actual web sites being loaded can still make a noticeable difference in the test results.
From the info you provided, your battery seems perfectly fine to me and within the margins of difference one would expect.
Best Answer
A common solution to this kind of problem is a complete restore (possibly in DFU mode). Before proceeding, make a backup of the entire content of your iPhone via iTunes or iCloud. Then, download the latest firmware available via iTunes. Charge the battery entirely before the restore, just to be sure that the iPhone won't shut down during the process. Proceed by putting the iPhone in DFU; let then iTunes do the rest. I suggest you to DO NOT restore your iPhone from backup after finishing. Test everything, including Wi-fi connection. Install an app that shows you the actual battery cycles. Maybe replacing the battery could do the trick. If possible, try to connect to different hotspot; if the iPhone could not connect, maybe another problem is in the antenna. Let us know.