As a long-term trackie, with many riders on Speedplays, I should perhaps clarify a small point here. There's no additional tension in the "track" design. The ramp from the float region on the pedal to the clip-out region is gradual on the road pedals so you can feel it and clip out smoothly; on the track version it's very abrupt so you bump against it more noticeably. This feels like more tension, but if you twist your foot you come out just the same and with the same amount of force applied in the twist, and you are out suddenly without any transition to warn you that you're in that dangerous range. Many track riders buy the "track" version because it's labeled thus, but switch to the road pedals (or warn others to do so) because with the road pedals you at least get the kinesthetic warning when you start to twist your foot. There's really no reason except a difference in feel -- no difference in actual tension or resistance to release. Many pro riders get fitted well and don't need float, so they like the abrupt feel with minimal float. That's all.
The Pro riders that use speedplay uses the fix cleat which very similar to the track version. This defeat the purposes of using speedplay except the pro have no choice due to sponsorship.
When I first read this I thought it was BS, but...
The difference between a regular Zero and a Track Zero is indeed only in the pedal body. The metal inserts in the body of the track version are gold-colored and have a more aggressive shape, that's why the release tension is higher.
Then I started looking at pictures of pro bikes.. and guess what: while they appear to be regular Zero's lot's of them have gold-colored metal inserts.
maxima seems to be right, nice catch!