Although I do not have any specific experience with the Durace and cycle dynamic combination, I do have some experience with cycle dynamic cogs and in making narrow chains. The following will be some of my observations to help you find a solution to the problem.
First of all, are both your shifters and rear derailer 8 or 9 speed? They should both be the same. How about your chain? Is it worn? Check to make sure that there aren't any burrs on the cogs, that has been a problem for them in the past. Are your 8 speed cogs road or mtb, the road ones have two shifter ramps machined in and the mountain have three. I don't think it will make much of a difference except if you mix and match road and mtb cogs. Once you know that these things are all okay then it comes down to cog spacing. Common logic would dictate if you have 8 clicks at the lever then you have 8 movements at the rear end, but if the spaces are not almost dead on to Shimano's design you will have the problems that sound like what your shop is talking about. So, your next would be to compare your cogs to a standard Durace set-up. Are the spacers the same width in every spot? Next are the combination of the cog widths and spacers exactly the same in every spot? Once you have your spaces dead on so that 8 cogs and 7 spacers in between add up to exactly X number of thousands of an inch give or take a couple, I can't see why there would be a problem. (unless there is some damage or extreme wear to your shifter or rear derailer). As to the allegations that a 9 speed chain would be slower it wouldn't be on the outside because a narrower chain sliding sideways on the teeth will never rub any adjacent cogs. It could only be if they fit tight on the teeth. So, take a 9 speed chain that you want to use and see how the narrow links fit on the chain ring teeth, do they fit tighter or slide on and off easier? The same goes for the rear cogs. I know or a fact that the cycle dynamic cogs are not exactly the same thickness through the whole block, they vary by over ten thou, and that's where the combination of cog thickness and spacer width comes in as I was talking about before. There might be some cogs that are too tight for a 9 speed chain. If your are set on this combination you'll have to machine or grind these cogs, once your chain fits then it comes down to spacing again, exactly duplicating the Durace set-up. So you might have to make some custom spacers. Even though a 9 speed chain is narrower, as long as it has lateral flexibility (evenly spaced) and no stiff links and slides on and off the teeth I can see no reason why it should be slower shifting.