Strange playability difference between Regal and LP

At the risk of sounding like a dog with a bone over something I'm not really that bothered about, how did you test the compliance of the strings?
you mean the tension required to get a particular pitch?  With a digital tension meter.  Pretty high tech for those days...
Not really no.

There's no argument that any additional string post nut, break angle or friction at the bridge will effect the tension required for a string to reach a certain note.

But they may effect the force required to bend/stretch the string, these guys use the term 'Compliance'
Hmmmm.  The counter argument to that is, if you're bending to a particular pitch, you're putting the same additional tension on the string no matter if there is zero friction or infinite friction at either end.  Same pitch = same tension = same bend feel. 
Thanks Mayfly for some substance in a land of roaming mythology. We either talk ourselves or get get talked into much internet folklore. I have to also agree with Cagey as he has a huge depth of hands on experience and has worked on or built a lot of guitars. Regardless of having tried to "design" in certain consistencies between the three parts casters I have they all feel different in playing in some way. The one Cagey built for me sustains for days, I love my other two as well but they are as unique as people in comparison despite following my own formula of strat platform-vintage style pickups-same components. (well, almost exactly the same) At an earlier point I had got talked into staggered tuners on one as required to forego a string tree but have the non staggered version on another (same brand and model tuners) and was able to forego a string tree as well. I have concluded its variations on a theme and they all feel different and have their own personalities within that variation.
With the same string gauge, the differences are somewhat smaller. The LP still plays effortlessly and faster in comparison.

The frets turned out to be 6105 on the LP as well (might not be typical "medium jumbo" frets but that's a "Slash" model).

So we're left with:
- fret material: nickel (LP) vs stainless steel (Regal). The Regal neck should be easier for bends.
- fretboard: rosewood with rough open grain (LP) vs ebony (Regal). It could be that the coarser rosewood has somewhat less friction (the ebony might almost be too smooth, like a finished neck in some ways).
- nut: here I could really improve how I cut the nut to have less friction. Worth a shot and easy to do.
- back shape: standard thin (Regal) vs Slim 60s (LP). That makes the LP a little chunkier, closer Warmoth's 59 profile.
- 10-16" radius mismatched to the 12" TOM bridge on the Regal.

I think the last three, and maybe differences in string material, are the most relevant.
Though, building from scratch another 12" radius Hombre neck is an expensive route compared to showcase necks, especially if the hypothesis is wrong ;-)
Problem solved.
I re-polished the frets to a fine grit, changed strings to what I think is the same brand/model as on the LP, and made a final adjustment to the nut.
Can't say which of these solved it for me since I did all of these in one shot, but as far as string bending goes it's just like the LP now. I might still go with a 59 neck as future upgrade, I think I'll prefer that over the standard thin contour.
Anyways, thanks all for the suggestions!
Cagey said:
amigarobbo said:
No, in principal the more it breaks over the saddle the less 'slinky' the strings should be.

There's no such principle, it's just a popular fallacy. A string's vibrating frequency depends primarily on its material, cross section, vibrating length and tension. Shorter speaking lengths require less tension than longer ones to vibrate at a given frequency. That's why when you have two guitars, one with a 24 3/4" scale (like a Gibson) and one with a 25 1/2" scale (like a Fender), with both strung up with the same string set and tuned the same, the shorter scale will feel more "slinky" than the other. There's less tension involved. Matters not how long the string is, it's the scale (vibrating length) that matters.

If overall length and breakover angles made a difference, then guitar designs like the Jaguar/Jazzmaster would be so slinky it would feel like it was strung up with rubber bands, since their strings run for several inches past the nut and bridge and at low breakover angles, compared to something like a Melody Maker, where the headstock is shorter and the bridge is the opposite termination point, both of which have high breakover angles. But, the shorter scale length of the Melody Maker means it'll feel more slinky.

I totally agree on the scale length but ...

have you've never noticed a difference, on the same Les Paul, with the same strings,
between the stop tail piece totally flush against the body and then raising the stop tail piece as high as possible?
flipping a strats hi E out of the string tree and retuning it up to pitch?

Both of those are outside of the vibrating range but they do create a change in 'perceived' 'slinkiness' ... subjective as it may be.

Don't make me pull out the 12 foot long table with the stop tail piece down there at 11 feet and the les paul attached to the other end with those 14 foot long strings.
Vibrating length would be exactly where it should be for a Les Paul ... but what would the 'perceived' stiffness now be?
I'm going to say the compound radius has a lot to do with it. When I make even slight adjustments on my guitars string saddles height the "bendability" changes drastically. If you had a set of files you could try and dial in a different radius on your TOM.