I have a geriatric girl with a murmur...mitral valve deterioration...a very different thing from what your pup has...but I found a website that discussed in depth heart issues/treatment options/and when treatment provided benefit and when it didn't/which meds provided the maximum benefit...course now I can't find it (but printed up the article so hopefully I can retrieve the web address and will list it latter). Anyway, in trying to locate it, I found this article on AS which you might find helpful..it actually sounds more hopeful than what I would have expected.."in about 10% of those affected, the defect gets progressively worse"...and "In the majority of cases, development of the defect does not usually progress beyond puppyhood i.e. the grading of the heart murmur would be unchanged from the age of 1 to 2 years throughout the rest of the dog's life."
Here is the link:
Canine Health - Aortic Stenosis (AS)
Also, my girl is now on ace inhibitors...Walmart and Sam's club have generic meds for $4, so if you decide to put her on meds, check there!