Tom,
You didn't mention what your guiding exposure is. If less than 2 or 3 seconds then, yes indeed, you will be chasing seeing. You also didn't mention if you find the resulting images acceptable or not.
One way to see just what is going on is to disable guiding but enable logging. Then you can go into the log (using Excel or Andy Galasso's Guide Log Viewer) and see just what seeing along with mount bearings + gears are doing before guiding gets into the act.
With a G11, unless somebody has over-tightened things (which happens a lot when users attempt to eliminate backlash), you should see a smooth trend line in DEC caused by polar alignment being imperfect, tiny errors in the tracking rate in RA, and atmospheric refraction. Superimposed on that you will see what the atmosphere is doing to you. When de-trended RA and DEC should look similar (from a statistical point of view). Having the guide camera oriented so the sides of the chip are parallel to the directions of axis motion makes it easier to interpret the results.
When you turn on guiding (with reasonable parameters - find the best usually takes a bit of systematic fiddling) you can then begin to look for problems. You may already be aware of them but some common tricks include biasing the mount weight slightly east heavy. That together with a guide rate of 0.5X or lower ensures the RA gear always stays in contact and never reverses. If you have a small amount of DEC drift then changing the DEC mode so it only corrects in one direction and (possibly) biasing the OTA to keep the DEC gear engaged on the same side of its worm will eliminate backlash in the DEC axis as well. Doing these things, at least, may reduce the damage caused by the seeing. Too much DEC drift, of course, and you'll get field rotation.
I routinely image with a much older (20th Year Anniversary model bought used) G11 that I upgraded to the precision (brass) worms and fixed few things prior owners had messed up (like over tightened worm bearing blocks). Otherwise stock and none of the wonderful enhancements Losmandy has made in the past few years. I get about 0.5 arc sec RMS on good nights with a 18+kg load. On bad nights, over 2 arc sec. 1100mm (200mm f/5.5 Newt) is my norm. On tough nights I pull in my horns and go to 400-800 mm with smaller scopes. On great nights, with bright objects, I'll image with a Barlow to get about 3000mm but those nights are few and far between and, of course, the higher f-ratio requires longer exposures.
My camera has 6.8 micron pixels. Really high end astro-cameras have 9 micron pixels, by the way. Why? Because the bigger the pixel (more area) the higher the photon counts which reduces the (unavoidable) statistical noise at the lower light levels. Unfortunately the non-astro sensor-makers have been in a mega-pixel arms race for the past decade or so. Fine for general photography or for lunar/planetary work.
General remarks:
Smaller pixels on the imaging scope, of course, together with long focal length optics, make everything tougher - seeing/guiding that look horrendous with 3.8 micron pixels at 1500mm might look just dandy with 6 to 9 micron pixels at 800mm. Smaller (imaging) chip sizes make things even more demanding since you'll have to enlarge the image more to fill the screen. For example, compared to an APS-C size (nominally 24X17mm) you will have to enlarge the image 50% more to fill the same screen since your chip is about 17X13mm.
With your rig a 3 arc sec RMS translates to about 20 microns. Double that (roughly) to get the blur disk you should see on moderately bright stars. So I would expect your stars are probably about 40 microns across, or 10 pixels with your camera. By way of comparison, the diffraction-limited blur disk will be about 11 microns at f/7 (actually, it will be slightly worse since the 11 micron figure does not include the secondary diffraction - 12 microns is probably more realistic). That figure is the absolute best you could ever see and it will never happen - to get that you must have perfect optics, perfect alignment, perfect focus, no scattering of light anywhere (including between the chip pixels and the cover plate of the sensor), no tracking or guiding errors, and no atmosphere.
So your images are probably 3 to 5 times larger than the absolute best. Not great but if they are circular not horrid, I would guess.
Best Wishes,
Mark Christensen
St. Charles, IL USA
