If Legolas Thranduilion was Laiqualassë of Gondolin, Tolkien did make some mistakes in LOTR, as Legolas desired better light for shooting at the Hornburg, and Laiqualassë was night-sighted. The Fall of Gondolin was also one of the Great Tales, and I would have thought something so momentuous would have crept into the LOTR. Glorfindel was not stated as having been Glorfindel of Gondolin, by Gandalf, but there were enough clues in what Gandalf said to Frodo about him for me to believe absolutely he was Glorfindel, Lieutenant of Turgon of Gondolin.
I would have expected at least an allusion to Laiqualassë throughout the story. However, with all the notes Tolkien accumulated over his life, I am sure he did forget quite a bit. He was trying to the end of his life to put the two Glorfindels together, maybe he just did not have the time for the two Legolas'!
The thing I really loved about the LOTR was purely its hearkening back to the Elder days. It was the book which lead me to the Silmarillion, but there was nothing in there about Laiqualassë, and Legolas considered himself purely a Silvan Elf and called himself that. I would have expected something more.
I do like the idea of both Legolas' being the same one, but Tolkien made correlation difficult. he said Elves don't recycle names, and then apparently did it himself, with enough background on Thranduil and Oropher for us to reach the conclusion that Legolas was a Silvan Elf, born in Mirkwood.
I've never read a story where the two are the same, although I would like to. The best short fic I have read which deal with it is
Legolas Reborn, on the stories section of the Tolkien Encyclopedia
I enjoyed that immensely.
I have to say though, my favorite pairing is Glorfindel/Legolas and in that series Legolas is very young. It works since the author is wonderful.
If any-one comes up with a good Laiqualassë/Legolas plotline and story I'd definitely read it ( if it were Elf/Elf slash ) d;-)