Kirk in TSFS is excusable. Kirk in TWOK is not. First, he thumbs his nose at a regulation that he well knows -- and that Saavik reminds him of -- and that is there to protect everyone involved. And for what reason? This isn't contacting V'Ger, where raising shields could be misinterpreted and cause an incident. This is a fellow Federation starship. If nothing is wrong, Terrell will understand why Kirk raised the shields: the regulation says to. If something is wrong, as it turned out to be, Kirk has safeguarded his ship and crew. There was absolutely no point to Kirk flaunting the regulation in this case.
But then, on top of that, Kirk ignores multiple warning signs and persists in keeping the shields down. First, Reliant transmits an explanation for their radio silence, which Spock's sensors determine to be a lie. No response from Kirk. Then Reliant raises her shields, which Spock detects and reports. Again, no response from Kirk. It is not until Reliant is literally locking phasers on the Enterprise that Kirk reacts and orders shields raised. At that point, of course, it is too late.
What Kirk demonstrated there was absolutely gross incompetence.
What I find interesting about that exchange is that due to the cut between the two ships the scene "feels" longer then it really is. It could be that the reports are coming one right after the other with almost no time to react. It would be interesting to see the scene without the cuts to the Reliant.