Hello! Sorry for the late reply. In short, Lethal Tempo does outscale Hail of Blades late game. However, you may find it difficult to fully stack Lethal Tempo without getting punished heavily.

Since Tristana has low auto attacking range in the early game it's very easy to get auto spaced by champions like Caitlyn or supports with high kill pressure like Thresh since you need to commit more time to pop Explosive Charge. There are only a few matchups that make Hail of Blades mandatory: Draven, Samira, Lucian+ Nami taking anything else into these matchups means pain and suffering for a lot of the laning phase.

If you find a lot of success with it you can totally stick with it (especially if you find yourself in a lot of 30+ minute games) but just keep in mind that Hail of Blades makes you harder to punish if enemies are playing well.