Specifically, I'm looking at 640kbps AC3 and 1500kbps DTS. Will there be a noticeable or detectable difference when someone listens on cheap consumer headphones?
The short answer is "No".
The long answer is:
I did a study in university to find out if people can, as some people say, notice the difference between different audio codecs and different bit-rates of those codecs. So I created a program in which the subject could rate 'versions' of a song for quality. In this program, each song would have it's own page, with a few numbered versions of the song listed down the page. Each numbered version would be cut up into sections for easy comparison. These different (numbered) versions would be different codecs at different bit-rates. The subject would be asked to play the different sections of each version as desired and list the versions in order of best general quality. I used high quality monitoring headphones to ensure all frequencies would be accurately demonstrated.
I can't remember the details of the results, but I remember that out of the thirty-something people I tested, only a few could distinguish between the original 44.1k 16bit wav sample and any of the codecs above 320kbps. The ones who could were mostly audio professionals or enthusiasts. The frequencies that are more noticeable in compressed codecs for various reasons, is the high frequencies.
The choice of headphones will make a much bigger difference to the quality than any bit-rate above 320kbps(stereo), assuming a good compression codec was used.