Are UMEP courses worth doing?
Do they drastically affect your ATAR?
Does it help with SAT applications?
1) Yes worth doing, ( i did UMEP maths) if you're interested in the subject and likely to do at at uni it gives you room to play with subjects, it's good for interest, and makes the VCE equivalent look much easier which can help with your VCE score, judging from from sig you look like you're planning to do it in accounting or biology, accounting is good, it will get it out of the way and you can do more interesting stuff in uni (granted i'm a little biased against accounting as i hated it)
2) Probably won't drastically effect you ATAR, especially now they've gotten rid of 5.5, but still boosting ATAR shouldn't be your only motivation of doing it. Having said that though, if you take it up and drop a subject it's probably an easier workload than getting a 40 in a 3/4
3) No idea, appinway who's at Yale now and who did UMEP physics would be a better person to ask, don't really think it would help great deal, even if you did do UMEP you would have to have a lot going for you to get accepted into an american uni