Lethality spec feels awkward with the reliance on shiv (4m) for TA. This forces the spec into melee without giving it any other melee advantages (gap closer, improved stealth, useful defensive CDs, etc). The only other option is to go up another tree (kolto probe) to get a ranged TA, which is hardly ideal. I can think of two possible solutions.
1. A high tier lethality talent that enables corrosive dart ticks to proc TA (similar to KP). Around 30% would be fair.
2.Alternatively, give overload shot a high tier talent that gives it a 50-75% chance of granting TA. To prevent spamming you could put this ability on a 6-8sec cool down.
My main is an assault vanguard (pyrotech). The class is a pleasure to play as the DOTs are easy to apply and have a 30m and 10m range. The nuke is 30m though needs either a 4m or 10m to reset its cool down. It means you can seamlessly go ranged to melee and back again, and always have an attack to use. This is how a lethality op should feel. At the moment, it just doesn't feel very fun.
Edit. The pre-empt the "Pyro is overpowered" backlash. Yeah, it feels overpowered against the weak classes for sure but against maras, juggs, snipers and assassins it feels fair. It's operatives (and mercs) that are underpowered.