If it's come on suddenly much more likely to be a broken ring, even with ace comp, depends which one is broken.
I had ace comp/mpg/emissions from my sapph but I knew the rings were totally shagged due to 200,000 miles!!, it burnt shit loads of oil, could use 1L in 50 mile hard use no problem in the end.
Stem seals can be done with head on car easy on these, why not do these first if unsure ?
tabetha