Jek Porkins:

"When you receive a stress token, you may remove it and roll 1 attack die. On a result, deal 1 facedown Damage card to this ship."

So I can use his ability so often I want to I guess? If a R3-A2 BTL stresshog stresses him, he can try to remove both stresses at once (step-by-step)?

But what happens when I use him with opportunist?

Opportunist:

"When attacking, if the defender does not have any focus or evade tokens, you may receive 1 stress token to roll 1 additional attack die.
You cannot use this ability if you have any stress tokens."

I don´t see a limitation here. But that would be probably bit broken (although very risky)

Everything has a "once per opportunity" limitation. So you can't keep adding dice with Opportunist as long as Jek survives removing the stress.

He can remove the stress the moment he gets it, but yeah there is a fifty fifty chance he can't hold it together. Opportunist doesn't keep stacking dice, but the upgrade works pretty well for him, gets some nice damage out if you feel like gambling....and they don't have tokens.

If you had a way to perform multiple attacks, I could imagine doing it more than once, but I assume it will trigger once per attack.

Sure it could work but again without the R5-D8 Jek makes the risk of killing himself. Then again there is also the other limitations for Jek to include the Dial, the limited firing arc, and the less than stellar pilot skill that will be exploited by Soontir and other like ship builds.

Also it all comes down to point efficiency, I'm sure you could spend any number of points and make Jek a fearsome ship however you probably would have spent less points making a standard Poe or Corran build.

Stay on target and targeting astromech an integrated astromech is going to be a fun build to try out. Only if he can hold it together.

Tried that in a specific list

it's a WORLD OF PAAAAIN

I'm telling you, it is!
TARGET LOCK PARTY!