Well, sometimes a Toyota tech will suggest premium, but it wouldn't have anything to do with a problem like this. They'll suggest something like that if you have a failing cat. Sometimes lucky few are able to skate by for another year after clearing the code and switching to higher octane.
I figured that it was mostly BS. It was almost free advice but i still expected a little more from the dealership, especially after working on it as long as they said they did, and the fact that the mechanic doing the work had the same engine and going as far as measuring the resistance on the coils
The only way the dealer will be able to simply give up is by either trying to charge you way more for diag than you're willing to pay and hoping you'll get it out of their hair, or by saying it's normal, or by claiming they can't duplicate your concern...which in this case doesn't sound like something they're going to be able to say.
I have considered this and is my worst fear, other than finding the actual problem is going to cost $3k to fix. Considering they have already done some diagnosing and I can't get a CEL to point me or them in the right direction, this seems a likely sequence of events. As a result of that, Im inclined to put up with the shop working extremely slow, and hesitant to raise my voice in frustration.
All in all, I would definitely take it to the dealer with much more confidence that the issue will be found, BUT be prepared for the potential for them to ask for your approval of more diagnostic time money if the tech is unable to figure it out relatively quickly.