I havent noticed a huge change in CPU since 1.7...I have a couple of FX laden kits that can hit the mid twenty percent in Live. Mind you Lives CPU percentage isnt completely accurate.
I did this little test just now, not scientific or 100% accurate of course...."real CPU" is taken from windows task manager
Win7 64, Intel i5 2.5ghz. Ableton Live 9 32bit, 44/24, sample rate 442
Vermona kit, no FX
Spark idle Live CPU 10%, real CPU 5%
Spark playing Live CPU 17-19%, real CPU 5-9%
Maschine idle Live CPU 3%, real CPU 2%
Maschine playing Live CPU 5-6%, real CPU 2-5%
Factory 808 kit with FX
Spark idle Live CPU 10%, real CPU 3%
Spark playing Live CPU 16-18%, real CPU 6-9%
Maschine idle Live CPU 4%, real CPU 2%
Maschine playing Live CPU 6%, real CPU 3-7%
I think it shows a big difference in Lives CPU report but the CPU reading taken from Windows task manager is far closer