Back in 06 when I still lived at
Mt Campbell Observatory 
I and a couple of other astronomers had the oportunity to directly compare views through a Meade LX90 a Synta 6" achromat, a 6" AP Starfire EDF and Meade 12" LX200GPS under rather good seeing.
Result:
The images through the 12" SCT were significantly brighter, faint objects were more easily revealed and under the good seeing the resolution on Jupiter was about equal to the Starfire but the brightness gave the impression that it was easier to see these details, at least this was the consensus (may be wrong of course).
The Starfire images were only marginally better than the 6" Synta, with very slightly more contrast and less colour fringing on brighter things (all expected).
The views of Jupiter through the Starfire were deffinitely crisper than through the larger SCT but dimmer. We could see red spot junior and detail inside the great red spot through both the Starfire and SCT but again the slightly better crispness was there in the Starfire.
Although the star images were not as perfect, we were able to split a 0.6" double star through the 12" SCT but could only manage 0.75" through the Starfire but they were two beautifull white dots with a single broken faint ring around each = optical perfection.
The views through the Starfire, as far as brightness of faint extended objects goes, were about on a par with the LX90 with perhaps a very slight edge to the LX90 when going back and forth between the scopes.
EDIT: actually I just went back and had a look at my observing log, of the three observers present, two thought the exrtended objects were marginally brighter through the LX90 the other thought the Starfire (not me) - ie it was that close. Aparently the double star splitting was better through the Starfire (I hadn't recorded what stars we looked at doh!) and all three thought the contrast was better through the Starfire. Worth keeping your observing logs, huh?
In other words a high quality (the best?) 6" APO is at least as good as an 8" SCT but provides crisper more contrasty views.
I imaged through both the SCT and Starfire and I have to agree with Greg that the overall results through the Starfire are superior to the 12" SCT even with less Focal Length.
These are just observations and far from diffinitive but they are good indicators of how the range of scopes compares generally from an observers perspective.
Mike