Well, I've had LT testing done. The protocol was a ramp test to vo2max in 25W increments, a minute at a time. So there is an issue of precision. I've also had FTP tests done using respiratory quotient. 20W increments. I think ramp tests are more reliable in one sense as they don't rely on pacing an effort. Many people will under test initially as they learn how to pace a time trial effort. You need to be motivated as it's not easy.
2009 LT was 225W. RQ testing last year went 220, 240, 260W. This year 260W, 280W. I compared the 280W against TrainerRoad 2x8 min test which scored 275W. So I guess really the RQ test said threshold up to 280W. The RQ test added value because it confirmed a big improvement in aerobic base fitness. I think LT testing can confirm this also as lactate should be lower in the subthrehshold zones with a good base.
The other issue to consider is just because your LT is X the length of time you can sit around that level is influenced by how good your base is, by motivation as mentioned above, and also by muscular endurance.