how helpful is being a legacy?

anywhere. It can and might help you if you "click" with your legacy group (They will give you a closer look. Every sorority has different ways to do this). It helps more with some that with others, but some chapters have more legacies than spots in their pledge class, and those will release quite a few. It does NOT guarantee you a bid.
It can actually hurt you with other chapters, especially if your legacy group is highly sought after and the others choose not to compete for you. Make it clear to other groups that you are interested in them regardless of your legacy status, especially in the later rounds.

Most sororities have an obligatory amount of rounds a legacy has to be invited back to or something along those lines. For instance, my sorority's nationals require us to give a legacy a bid if she makes it to preference round. However, we have cut legacies before preference so we didn't have to give the girl a bid. If you have decent grades and an ok reputation, coupled with rec letters stating your legacy status, most houses will probably keep you around. If you have bad grades or inappropriate social media, most won't. It's usually no more than a cushion to put you on the radar.

The legacy thing is tricky. I went into rush with a preconceived notion about my mom's sorority. I did not want them. I ended up really liking them during recruitment, and she got her hopes up. They cut me before pref. It was the only sorority at UT that cut me. I didn't plan to go with them anyway, but my mom was extremely hurt, and has a bad taste in her mouth ever since, She feels more connected with my sorority than her own. I hate my mom was hurt, but I was actually relieved. It's not my fault:-)

Moral of the story... go where you feel comfortable. It is your experince, not your mothers. She will be like mine, and just glad you have a sisterhood and friends regardless of the letters you wear.

Chi omega cuts legacys with excellent grades, excellent reputation, volunteer hours, multiples awards etc., even when XO has asked the girl if she would pledge. There is no reason a legacy should be cut with this type of Resume (RIF)

probably has more legacies than openings in their pledge class, and most of them also probably have the qualities you listed. About half the chapters at UT are most likely in this situation, so some legacies must be cut. This happens at all SEC schools, not just here.
Unlike previous generations, almost every PNM's mom went to college and a lot were greek, so there are a lot of legacies coming through every year.