Everyone thinks they will fly in the marathon going from altitude to sea level, but I am skeptical.
The formulas say you can expect anywhere from a 3 to 7% improvement, but I have also found that weather at SL can be a confounding factor. Went from over 6000 feet to Dallas (a few feet) in December. The humidity got me (tornado warnings were in effect the day before the marathon). Oh and 7% would be really amazing!
Had a group run the Honolulu marathon (about as sea level as one can get) from Colorado and Wyoming. December--again the cold to heat did them in (a few did an acclimitifffzation protocol but still suffered).