My understanding is that Canon's HTP (highlight tone priority) really does change RAW values, and I've also read that it is just digital, so leave it off.
IIRC, the DPReviews of Canon DSLR's over the years have indicated that HTP does what it is supposed to do ... change the sensor's RAW response curve to increase RAW dynamic range at the cost of some noise (ISO 100 vs ISO 200).
I wrote to Canon tech support, and their reply was that HTP changed the sensor response curve for RAW, not just JPEG.
My experience from watching "blinkies" is that HTP increases the dynamic range, but that may be an artifact of JPEG processing. I don't recall if I've tested that since MagicLantern (ML) implemented RAW histograms and RAW blinkies.
My question came specifically in a OP to the ML forum asking about Dual-ISO and HTP:
I consider the ML developers to be true Canon gurus, yet I find myself in disagreement with them on HTP. However, it is possible I misunderstood a cryptic reply.
BTW: my understanding of ALO (auto lighting optimizer) is that it is just for JPEG, and doesn't make a difference for RAW. Or not?