There are degrees of sentience

Yep. And this also implies that once machines are above zero on the sentience scale they will continue up and will move past human-level sentience just like they moved past human-level chess playing. It's actually worrying ethically because a super-sentient being could experience extreme feelings compared to us, including extreme happiness and sadness, extreme psychological pain, etc.

I doubt current architectures will allow for a snowballing of sentience due to the power inefficiencies of silicon compared to biology. We are more than three orders of magnitude more efficient than current computers…