I have found at times that you can do a tempo run and be breathing every 4 steps (1...2... 3...4...)
(inhale) (exhale)
but you are probably going a bit too fast. Sometimes breathing and fatigue are not always coordinated. I have been breathing real hard over the threshold and yet my legs felt fine when I expected acid to start filling them up.
On the other hand, sometimes my legs are tying up and the breathing is regulated.
I think you should try to have both correct on a tempo run, and even if you need to keep it slower than ususal to have the right breathing and stay sub-LT on a tempo run, it is better than exceeding t and defeating the purpose of the effect you are trying to achieve.