Memorizing sight words did not help my ds, as he can memorize lists of words in minutes. His teacher used to give him lists of 20 words to keep him busy. He was not making connection between those lists and the same words in the text.
I have similar issue with spelling lists and my other ds, he has to read his spelling list only couple of times before he has memorized and can spell all those words. Easy As on spelling tests, but if no further connections are made, he will not be able to spell many words from those lists in few weeks� time. It has been up to me to work with him on word roots and prefixes.