What's the exact timing of this push? I'm curious because I've been doing my own unique variant without the +1 attack, and with stim instead of combat shields. My push leaves with 21 marines + stim at 7:25 gametime. If you can squeeze in a +1 weapons upgrade and still leave the base 25 seconds earlier than me, I might have to switch builds =P.

For the zerg out there: the proper counter to this strat is banelings (you need about 8), or mass spinecrawlers (4+). Pure massed roaches can work too, but you need a LOT (I believe 15+).