Ok, here's the update.
Install was super easy as expected, and the only difference I encountered was that the bridge '59 was 4-conductor. Red and white were already soldered together and heat-shrunk together, so black went to the pot lead and green+uncovered wire went to ground on the pot itself (instead of the braided shielding per the stock Gibson pickups).
I left the original owner's pole-piece setup on the 59s, and didn't take much time to play with the height.
Tone. I recorded some clips pre and post-install, but I need to cut them down from ~1 minute to a 5-10 second best-fit clip.
The bridge 59 is much less nasally compared to the 498T. Each string has more clarity and a little more openness and sparkle. Highs are no longer a problem, and while the mids stand out a little bit all the strings are voiced evenly. Absolutely no problem rocking out; these guys have half the output of the stock pickups but do just fine with the gain up. There's definitely more texture to the sound with an overdrive pedal, and less fuzz.
The neck 59 is also very clear, glassy, with lots of string definition and sparkle. With the stock setup I noticed some slight muddiness on the A-D-G, and a little boomy bass. I've since tweaked it slightly and the bass is absolutely not a problem anymore, and the mids are getting better.
Listening to the clips at home, you can hear where the stock 490R/498T are just itching to overdrive -- much thicker individual string tones. Whereas before I couldn't even play single note runs on the bridge pup clean because of the nasal sound (let alone chords), it's a much more pleasing and musical sound now.
And the center setting using both pups... wow. They balance each other perfectly here.
Overall, still gonna need some time to adjust them but I'm already extremely happy with the change.
Any opinions on what I should ask for 4 stock Gibson pots w/ caps and a 490R & 498T w/ chrome covers?