You can implement BDD with TestComplete by combining the Connected Application in C# and SpecFlow.NET, so you write your automation steps using TestComplete directly in Visual Studio. So far I have seen impressive results.
You need to use automation driver with SpecFlow.NET and TestComplete fits the purpose.
There are challanges as Connected Application documentation is not clear or scattered, so you need to spend some time in trial and error until you get confortable with the API.
Also there are few casting issues which might create noise with code check tools like ReSharper or FxCop.