Hi Buqpa -- First thanks to Doug for the nice comment above. I am on travel right now (at EuMC in Manchester, if anyone is there, stop by and say hi). Internet access is difficult sometimes.
It should be possible to get a reasonable answer from most any of the simulators. I don't know why there are some problems. I would guess that CST has a problem because it is a resonator. A high Q resonator impulse response lasts a long time and requires a lot of analysis time. But they have ways to handle that kind of situation well. You might find it in their documentation, or contact their support. If you are in North America, you can contact us too.
I have never seen any benchmarks on Ensemble and know very little about how it works, so can not comment there.
The problem sounds ideal for the free SonnetLite (I work for Sonnet). You can even use our fast sweep (ABS, Adaptive Band Synthesis) which is default. Circuit resonances have no effect on the number of frequencies ABS needs, in complete contrast to all other fast sweeps that I know about. It is really amazing to watch Sonnet interpolate data from 4 or 5 frequencies into an almost exact frequency response of a 20 resonator 5% bandwidth filter.
If you can get your circuit into AWR, you can then send it directly to SonnetLite. Also, the in the next release, SonnetLite will interface to ADS for free. So if you can get it into ADS, then you can get it into SonnetLite.
If you have any problems, post them here, or better yet, post them on the Sonnet User's Forum,
www.sonnetsoftware.com. It is seeing quite a bit of activity lately.