Time Machine is the software that runs on your Mac so that won't change at all with the new device.
I would run a few manual backups from the command line on your existing Time Capsule to see what amount of data you are backing up and how long it takes:
date; tmutil startbackup --block; date
Once you have a feeling for what your speeds are, you can start to determine what the bottleneck is. If your Mac takes too long to send the files itself due to a busy filesystem and CPU - no external upgrade will help. If you are waiting on the hard drive in the Time Capsule - that too is the same technology and you should expect minor speedups as the controller seems to be marginally faster in the new Time Capsule. Only if your bottleneck is the network speed between the Mac and the Time Capsule will the faster network make a difference for the time spent backing up.
The new antennas are working very well for cases where we have one or two machines at the edge of the wireless range - but we don't have many of the new Air that speak 802.11ac yet so although some speedup is possible - it's not as dramatic as the benchmarks show except in some limited circumstances.