Well I did run some tests last night with the kit I mentioned above, and like I suspected, it struggled. I took a series of 4 min unguided exposures then a series of guided exposures. I had the scope pointed east and fairly low when I started. The unguided run was first and I let it go for about an hour. Then I switched guiding on and let it run for another 3 hours. Unfortunately, I lost the phd guide log. I'm not sure if it maybe got saved somewhere else.
Here's a link to an album I created that has one typical unguided and one typical guided from each series:
As you can see, the unguided image has serious trailing as the mount is struggling to keep the speed going. Guiding fixes that, nice round stars. But even guiding was not all that great last night, the rms error varied between 1.2 and 1.6 arcsec, considerably worse than when I was running just the camera and lens. I didn't think the scope was that badly unbalanced, but it was apparently.
Still, the end result shows that if you do guide, you will do ok. If you do unguided with this setup, you might get away with shooting 1 min exposures, or maybe use a second counterweight to balance. I didn't try those two.
Btw, I did find the phd log from my previous night's run, when I was doing just the camera and lens. I ran PECPrep, is that what you guys use to estimate PEC? I'm not sure that tool is working right, it tells me the PEC is less than 1 arcsec, but how can it tell from the mount corrections? I wish PHD2 had a tool that just observes star movement and generates a PEC estimate based on that. That would be the most accurate way.
Anyways, for anyone interested, here is my guide log from that night.
If you figure out how to use it and get a useful PEC value from it, please let us know.
Hope this helps.