SARPE can widen the cheekbones a small amount. It won't be very perceptible in adults though. It is more perceptible in children which is where most of the studies are done . Likewise with the slight clockwise rotation of the maxilla that SARPE can cause.
Yes it can lead to unwanted fullness in the lower cheek area. You should inquire as to whether your lower palate is wider than the upper. If it isn't, SARPE can throw off the balance. SARPE can also throw off the harmony of the face if the upper and central midface aren't wide. I've seen this multiple times. Also, SARPE won't widen the mouth itself to any appreciable degree (if at all). Obviously the width of the mouth is important because it signals the width of that part of the face. If you're going through with SARPE you should be willing to seriously consider widening the mouth. If not, again, the net benefit will be very limited.
Whether a narrow palate is unattractive depends on how narrow it is. Once you've hit a threshold level, it decreases dramatically in relative importance (just like most other maxillofacial strands of beauty). If your palate is very narrow, SARPE is worth it despite all of the above. That is because a very narrow mouth is very unattractive
