I may not have the exact years right here, but I understand that the '00+ throttle body has better flow than the '96-99 TB, due to less restriction in the throat. Is this true?

I do know the MAF sensor is different between the years (though the connector looks the same); are the other sensors the same? The TB gasket is the same, so I assume the connection at the UIM would be the same...what about the connection to the intake "accordion tube"?

So, what I'm asking is, would a '00+ TB bolt right up to a '96-99, assuming the MAF was also brought over? Would it improve the intake?

Its the 99+ that dosen't have the maf post. It has a hump where the maf protrudes.
It will bolt up but I'm not sure if you will need any additional tuning since the mafs are physically different. (you can't use the 96-98 maf in a 99)
The 99+ does look more free flowing but by how much, I don't think anyone knows.
Had to change it for a reason I guess.

I've done some quick and dirty WOT volumetric efficiency testing comparing my nearly stock '98 LeSabre against my wife'* totally stock '04, and I get 75-76% VE, while she gets 81-82% VE. While I haven't cleaned my TB/MAF lately, I'm thinking that alot of that difference might be the improved TB on the newer car. I'd love to be able to swap the TB'* around just to test that theory.

There are many variables that can effect the volumetric efficiency. I doubt the TB is a major contributor.
The pcm is probably a major one as is the general mileage difference.

Agreed about the many factors. That'* why I'd like to test a direct swap, to control for those other factors!
(Unfortunately, the MAF'* must also be direct swapped, so any such experiment can have that as a critique, no matter how carefully all other factors are considered)

The MAF sensors (and thus the MAF tables in the PCM) for the various TB'* are different. Unless you could find a way to put the 99 MAF sensor in the 00+TB while still maintaining reliable readings (or change the PCM to use the MAF tables of whatever MAF sensor you installed) I would say no.

99 maf is the same as the 00-03.
One thing I forgot about is in 04 they changed over to drive by wire. So that makes it even more complicated if you were thinking of swaping over the TB. It woudn't really be worth it to adapt it because of the differences in inlet to the UIM let alone the fast its electronic.

Drive by wire in '04?

Dang, I'll have to get under the cover on my wife'* engine and look at that. I swear it has the standard two cables leading to the TB (throttle and CC)...

I notice her '04 has AWESOME throttle response compared to my '98...could be partly due to mileage difference, TB difference, my need to adjust TPS, deferred maintenance on mine, etc....but could also be due to this drive by wire.

Actually, I'm probably wrong since your rpo says L36 vs L26. Maybe it was 05 for the Lesabre.