Struan, you have an interesting camera set up. I don't want to seem negative about the use of the Auto Ap shutter but after extensive testing of my Auto Ap shutters, a shutter system I have used daily for more than a decade,and as compared to using a standard Copal shutter I found the Sinar Auto Aperture unacceptable for slow shutter speeds with long lenses.
I mounted the SAA ( the f4 model, I also own the f 5.6 model) to a Sinar P2 4x5 and shot film using the 300mm APO Ronar, 360mm Sironar-S and 480mm APO Ronar. The tests included mounting the camera to a Gitzo 509 tripod, center column removed and base plate used, and a 9' tall 300 pound Foba Astia camera stand, Sinar pan tilit head on both.
With a 8x loupe there was a noticeable difference in sharpness at slower shutter speeds. Now maybe the greater mass of your rig, 8x10 size( which will require less magnification and therefore shake becomes less visible) and with a very large and heavy lens such as the one that you use, it may have a dampening effect on the shutter's vibration, but on my rig, with no customizing done, that is stock Sinar components assembled in the exact format required by sinar, on a Sinar camera, I had shutter shake. Then again what I consider unacceptable may be acceptable to others.
The shutter is designed to automatically close the aperture, open and close the shutter when you add or remove the film holder, and cock itself. If you do not use a camera that is compatible with Sinar lensboards,GG backs and bellows, you lose these functions.
As for the need to carry spare Sinar cable releases it's true that they are hefty, but i've seen many of them break in studio conditions. Counting Elephants will work fine if you tend to do long exposures, as in a few seconds or longer, however it doesn't work so well when you need a 1/8th or 1/30 sec shutter speed. For these reasons and others I decided to go to the added expense of buying copal shutters for my lenses and only use the SAA shutter in the studio.