I agree that the "problem" with the Defender is not the fire power or the dial. Honestly in most cases stress is not something I worry about when I fly mine. The oppertunity for a tactical green bank always seems to present itself with proper piloting.
The problem is twofold:
One is the over all defense. In truth the evade action is not something I would use often if i had it. Focus is the way to go. If the dice hate you then the dice hate you. The shields and the hull are what they are to tank that.
So in light of that how about:
Modification: Secondary shield Generator
TIE Defender Only
Add two more shield tokens to the ships pool.
Cost: 4
This is not unheard of in the Star Wars universe. This type of defense was made famous by the Mon Calimari.
Game wise this keeps it from being super powerful in the firepower area but gives it a tough enough skin to actually seem like a super advanced superiority fighter.
The second problem is the perception people have of the Defender.
The ship it self has so many potential upgrades and synergies that it's almost always worth the cost. In a two ship build you get a lot of HP, anywhere from good to staggering fire power, and missles to boot just in case. Take the 'Spray, loose the agility in both dice and movement.
In the lore, even Rogue Squadron found them to be formidable fighters but not without their flaws. I think with a little bit more skin they would be just fine.