Apple's QuickTime site states that the recent QT 6.5.2 release "...includes enhancements in the Apple Lossless Encoder and AAC codecs..."

anyone have substantive info on what these changes are wrt Apple Lossless? any observations about encoding speed / resulting filesizes / other ?

Apple lossless now finally deserves it's name: Now it is GAPLESS !!!
I just tried it.

How did you test it?

I converted WMA 9 lossless files with iTunes 4.7 to apple lossless ... converted these to WAVs and burned an AUDIO-CD with Instant-DVD/CD. I use "Robert Rich" "Bestiary" as Test-CD (all titles crossing over).
iTunes failed while burning the CD itself ... there are little gaps on the Audio-CD!
Gapless also works when ripped by iTunes and burned externally! But I would never trust iTunes for Ripping Lossless ...
A pity that creating Audio-CDs with iTunes is buggy ... because it is so easy!

QT 6.5.1 would not decode the last few samples (up to around 100 samples were missing). This is now fixed.

And it has become a little faster: