As I understand, the L26 is the Series III 3800. It has an aluminum top, and there are probably a few other changes. What would it take to swap my SLE over to the L26 UIM (and LIM), and would do anything for performance considering GM must have improved something to warrant the change from plastic to aluminum. I have read a little about this elsewhere, but I believe nowhere can I find any info as in-depth as I know we can type out here on BC.

I don't see the performance benefit...both the S2 and S3 are near identical in that regard. Not only that, you would have to adapt the drive-by-wire system to the L36. Meaning you lose your current cruise control since it no longer have a use. Which also means a full-out harness and PCM swap.

I'm crazy, but even I wouldn't attempt that.

You can make it mechanically work fairly easily but you loose the PCV functionality and end up with a miss matched TB to UIM neck.

You need a Gen V adapter plate and you need to block off the L26'* PCV and possibly IAC.
That adapter will mis match the TB vertically and the butterfly shape of th L36'* TB exit looks nothing like the L26'* round inlet..
A L67 TB would match closer, although not perfectly, and tuning would still be needed.

It'* been done a few times but I see a performance/functionality loss if anything..

Very good. Thanks so much for clarifying!