[pfdietz]

High Isp is actually a bad thing in a first stage. That stage is done almost immediately, so high Isp doesn't buy much, but low propellant density (which tends to go with high Isp) makes the stage larger and more expensive. This is why launchers tend to use hydrocarbons, not hydrogen, as fuel in the first stage. Low propellant density also makes the pumps larger and so reduces the thrust/mass ratio of the engines.