Not only would it need to be sustained, but fading would need to be soaked.
Say the fading is level + 1, to get on par functionality of +2 dice, they'd thread at level 2 and need to soak 3 fading. Say they have 6 res + 5 will that's 11 dice, so on average, the TM should be able to hit the fading threshold on average, and take -2 to all other actions for sustaining.
But lets say, we have 6 software and 6 resonance, so average of 4 hits. So we thread at level 4. We need to soak 5 fading, which odds are we won't hit. But we do get +4 dice while shooting, and -2 dice to all other actions and probably take 2 stun. That actually sounds too unfair to me. I think the fading will need to be lowered, considering the sustain cost. -2 to all other actions is pretty damn brutal, but the possibility of getting lucky on an edge roll and getting something like +10 dice on shooting tests would be amazing. And that's kind of the point of edge is to do amazing things.
And, there is also the problem if the TM needs to hack his own gun. He'd have to beat his own int + firewall, which could be very problematic as the better his Matrix defense, the worst this bonus is. I don't like the sound of that, but if the TM doesn't slave his gun to anything, it only has a firewall of 2 and makes it easier for a decker to brick it. Which would add a very interesting risk vs reward.
And just because you can shoot better doesn't mean you'll be a great combatant. TMs still have problems with meat initiative, physical attributes, and because of the attributes; dodging. Also, something like improve ability for adepts is even more broken. So I think its fair.
Compare a complex form that increases social dice while sustained vs a mage casting improve charisma or adept's stacking all their powers to be a face. TMs attributes actually synergize well with being a face but they currently require too damn many skills to be viable at it and their complex forms and echos don't synergize with the face role. 6 hacking skills, 3 resonance skills, and 6 face skills. Any TM is going to have a hard time to be a face and hacker, so giving them tools to either hyper focus that or tools to allow them to branch out more is the answer. I do think they should have uncapped bonus dice on being a face with a complex, just so they can focus on being able to be hackers without needing to dump a ton of karma into social skills.
And I don't think any of these should be echos, because there are too many echos that seem mandatory and I'm sure that's just going to get worse. You can only submerge so many times before the cost is so prohibitive that you'll never be able to feasibly do it again. Complex forms should be what gives TMs flexibility not echos. Echos should reinforce the TM's playstyle as an advance option. That's why I think complex form skills is the way to go.