Re: why is there so much tension on some guitars?
I never heard back, but I'll take a stab at it based on the limited info in the first post.
First of all, looking at measurable tension, are the strings the exact same brand/model/gauge? If not, then one set of the strings may have higher mass (usually from a thicker core wire), which will have higher tension.
Looking at perceived tension:
How does the break angle from the bridge to the tailpiece on the Tempest compare to the break angle from the bridge to the Bigsby on the Gretsch? The one with the steeper break angle will play tighter.
How much string length is there from the bridge to the tailpiece on the Tempest compared to the string length from the bridge to the Bigsby on the Gretsch? The one with shorter string length will play tighter.
I can give a tip to ease tension on Bigsby equipped guitars: I went from the factory 7/8" spring to a 1" spring on my B5 equipped Tele. The longer spring has more give and makes the guitar play slinkier. It also makes the bar sit higher, which works better for me in an ergonomic sense.
How big are the frets on the Gretsch? Bigger/taller frets tend to play slinkier because you get the semi-scalloped neck feel. i.e. your fingers don't touch and drag on the fretboard like it does with shorter frets.
How do action and relief compare on the two guitars?
There are other possibilities, but that's a few as a starting point.