Yes, the proBNP will tell you if there is evidence of heart damage. I do believe that meds will help not only extend his life but improve the quality of life. I had a cat with a heart defect that was diagnosed in congestive heart failure at 18 months of age. He was not expected to make 2 years of age. He died at 12 and a half years old of lymphoma after having over ten years more of great quality of life. I actually hadn't realized how his heart condition had impacted his quality of life until we finally got it well-managed (which took a few years), he became more of a kitten than ever before. Because Savannahs are naturally high energy cats, any decrease in activity can really go unnoticed. And more subtly, when his heart condition was not quite "right" his temperament became slightly more on edge and a bit "freaky" and easily startled. I imagine it is how I might feel if my heart wasn't feeling right..
My Bobo is not the only Savannah I've known over the years with heart issues, and more specifically I've known of SVs with HCM. Donna Lawver of Summerwood Savannahs had an F1 diagnosed with HCM before my Bobo got ill, with careful medication and monitoring, her Guy lived another ten years or more with HCM. She might be a good person to contact to learn more about living with a cat with HCM?
I hope your boy is easier to medicate than my Bobo was... for the longest time he acted like we were trying to kill him every time we pilled him (twice a day). Eventually what worked was pill pockets. Not in the usual way of getting them to eat the pill as a treat, but to coat the pill so when we pushed it down his throat there was a pleasant aftertaste. Then it didn't seem to be so awful what we were doing. And then eventually (after more than a year of that) he actually would eat the stuffed pill pockets.
In terms of being in heart failure, my Bobo was in congestive heart failure three times, and kidney failure twice also. Heart disease can mean balancing the heart and the kidneys...especially in his case taking lasix then spirolactonone (diuretics) along with enalapril and pimobendan. It can be so scary, I found it heartbreaking to go through and am so sorry for you. This is the reason I have all my breeding cats scanned before using in a program, to try to avoid this kind of thing happening :-(