Microseconds are not enough, because the arbitrage works by the HFT setting the order in one place, and then rushing the arbitrage to the other place. As long as you're more coordinated than speed of light between the two locations, you're fine.
Also, GPS clocks are cheap and precise, atomic clocks seem gold plating. (Which sometimes is actually necessary in electronics, BTW...)
I think the main innovation in Spanner is the notion of Truetime not the atomic/gps clocks though. In fact MSR has come up with something called ClockSI that does global snapshots with a modified NTP I believe.
Also, GPS clocks are cheap and precise, atomic clocks seem gold plating. (Which sometimes is actually necessary in electronics, BTW...)