You'll likely get the same speeds since NBN on HFC is using the same lines as cable, but the difference is they'll cap you at 25 unless you pay for a better tier. It's extortion as far as I'm concerned and it's why I haven't moved to NBN either. I've got less than 3 months to do it as well.
I don't want to move to NBN when my cable connection is more stable and faster than I'll get on cruddy nbn
I got an email earlier this week saying they've bumped up the speeds......... similar to a few months/a year ago when they did the same thing. The speed has stayed about the same.