Let's forget about locking nuts for a second.
You're mis-informed if you've been told that tilted headstocks create tension on the string. String tension is a function of string gauge, pitch and scale length, not headstock angle. (Why do you think Les Pauls have a looser feel than a Strat? It's not the headstock angle, because by your argument, a Les Paul should have more
tension than a Strat.)
In traditional guitar design, the headstock is tilted to ensure all the strings break over the nut at a consistent angle. The breaking angle is necessary to create the downward pressure that holds the strings in the slots of the nut. Too little of an angle, and they jump out. Too much, and they bind and go out of tune. The angle increases pressure on the nut, not tension on the string.
Fender-style straight headstocks need to use string trees, staggered-height tuners or a retainer bar to hold the strings down in a slotted nut.
Floyd Rose locking nuts have the breaking angle designed into them. It's irrelevant as to whether the headstock is straight or tilted. It works the same because the nut is sloped from the fretboard side towards the tuners.
The Floyd Rose string retainer bar was originally designed for straight Fender-style headstocks, to hold the strings at an angle so the nut pads don't push the strings too sharp when they're locked down. It's not a device to alter the string tension on the playable part of the string... it's there for tuning convenience. On guitars that have sufficient back angle on the headstock such that the strings exit the nut flush with the bottom of the grooves on the tuner side, the retainer bar isn't even necessary.
If want to believe that having the retainer bar cranked down increases the tension on the playable part of the string with a locked nut, that's your perogative... but the mechanics of it don't lie.
Now, with regard to your trem-setter comment, it's like comparing apples to oranges in this situation. It's a totally different mechanical relationship with the strings compared to the retainer bar, because the bridge is a moving part.
A trem-setter does
alter the feel of your guitar because it's stopping the bridge from sagging forward when you bend strings, which means that all of your bending force is going against the string, instead of some of it pushing the string and some of it pulling the bridge forward.
And no, i don't think a JS
has any less string tension than a JEM with the same gauge of strings installed and tuned to the same pitch.
Go read the back of a pack of strings, where they list the string tension along with the gauge. They don't list separate tension measurements based on the kind of headstock, do they?