Interested in trying this as well. We have always burned through the goals with at least 3 buildings left, and I feel that adding another 5 minutes of gameplay would really be a good thing. More goals, more points, more rounds to expand. Seems like a good idea all around.
One possible reason why Laboratory wrote the rules the way they did was to help boost replay-ability. By only having 5 of the 9 goals present in any game, there exists the potential of never knowing whether a goal you were building for would come up in that game.