I know we are talking about Sonar, but to make solving the issue easier: What happens in other audio applications (like Reaper) with your setup?

Panu:

I am running Windows 7 (64 bit), Sonar 8.5.3 (32 bit) and two FF800's with no problems.  I am using the latest production FF firmware v 2.77.  My hardware is an Asus M4A89GTD PRO/USB3 mobo with AMD Quad Core Phenom II X4 965 Black Edition 3.4 Ghz processor, 8Gb Corsair Dominator DDR3 memory.  I mostly record classical ensembles and acoustic music in the field using up to eight mics/tracks simultaneously through the two RME units using the on board preamps.  I mostly record on location with the two FF800's on a Dell M1530 laptop and then bring the raw tracks back to the studio to mix and master on the Asus system.  I have been very happy with the FF800's and Sonar works well most of the time except for the odd anomaly that just seems to be part of Sonar these days.  But basically, everything is working very well.  Have you tried the v 2.77 drivers.  I have successfully worked with some of the Sonar demo files that contain a large number of tracks, some fully loaded with a number of effects and soft synths, and my system has not really even broken a sweat yet. Hope this helps. 

I only open Sonar for testing so I did not check the Arming delay yet, but I do have a delay of about half a second when starting Playback in WDM mode. Not so when using ASIO, and not when using WDM in Reaper.

What are your buffer sizes set to in Sonar?  Have you tried dropping back to v 2.77 of the firmware?

I am also using driver ver. 2.992.  I am not read of any particular problem with your Asus motherboard.

Tests with Fireface driver 2.992

* First time arming whole track folder containing 22 mono & stereo tracks hanged Sonar over one minute. After arming recording did not work - nothing came in.
* Firefaces power off & back on: now armed track, recording and playback work perfectly.
* But unarming track folder hangs Sonar again. After one minute of waiting tracks get unarmed but audio engine does not work anymore.
* Note: There's no delay in arming tracks when project is loaded. Delay comes only when arming is done manually.
* Note 2: I turned power off from Firefaces when Sonar was open and audio engine dead. Sonar found Firefaces again. Arming invidual tracks was super-fast but no track could be unarmed! After a short while Sonar crashed with error code c0000005.

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Fireface units:
1: Revision 1.00, bar code 22376811
2: Revision nor marked, bar code 22393798

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Sonar version 8.3.1

* No delay in arming all tracks at once. Recording works perfectly. I was able to record 7 x 22 mono tracks before dropout (154 tracks playing at the same time!)
* Unarming: no delay. And audio engine is still alive.
* BUT: Shortly after that audio engine hanged. I restarted 8.3.1 and made a short test recording. Some tracks looked like [img align=this]http://www.avaruusankat.com/etc/RME/failed_audio_recording.jpg[/img]. After restart all works again.
* I tried to arm and unarm all 22 tracks *) as many times as possible. First four times were OK, fifth time arming lasted over half minute. And after that audio engine did not work anymore. *) Two units were in use but all inputs were from 1st unit.

* Quick arm / unarm test in the same project for Sonar 8.5.3. 
  Unarm 1 OK.
  Arm 1 OK.
  Unarm 2 OK.
  Arm 2 FAIL. (Sonar not responding for two minutes).
  If I start playback it can't be stopped (and no audio comes out).
   Additionally, after this Sonar crashed again with Exception code c0000005.

Conclusion:
8.3.1 is more reliable than 8.5.3 when using two Fireface 800 units but it's not entirely crash-proof.

Coming soon results with Reaper...

I installed 64-bit version of Sonar 8.5.3.  It does not hang the same way as the 32-bit version does. Arming one track takes half a second but when playback is on arming is 10x faster(!). 
I can't make audio engine tilt.

All my problems are 32-bit Sonar related.

Sorry to hear of your problems, but at least you have a work around.  It is possible to run two versions of Sonar on the same machine.  You could run Sonar 8.3.1 (32-bit) with your 32 bit plugs and Sonar 8.5.3 (64-bit) with your 64 bit plugs.  I know it is not ideal and there should be a reason why the 32 bit version does not run.  Are you sure you have everything exactly the same between the different versions you tested?

Did you try using JBridge instead of BitBridge to load the problematic 32-bit plugins into the 64-bit version? What happens when you disable multiprocessor support? and/or MMCSS?

Since you configure JBridge via the Plugin Manager you at least need to have the plugin listed there. But it also allows you to add plugins manually and some other magic that might help you find the plugins.

JBridge is acting as a separate host and transfers data between itself and Sonar via Inter-Process Communication (IPC). That means that your 32-bit plugins *are* running in a 32-bit host then and just send their data in and out to Sonar. You could also use something like Port Audio or Jack to do it all manually, JBridge is more about convenience.