Having feedback from L+1 or +2 peers is very important, otherwise it can be very hard for the committee to judge performance.
Actually "leading" a project less so - often a single "project" can be viewed as multiple components / projects for promo purposes, what counts (depending on level) is initiative, leadership, and technical contributions. It's not necessary to be the formal lead of a big project for this purpose, but rather for your senior peers to attest how well you did in those areas and how it helped the project launch X months faster, improved QPS by X or whatever metric.
As pointed out in other comments, it can be hard for the committee to see impact, so the more hard evidence you have (before/after resource usage, bugs fixed, etc) the better. Same for improving the codebase, document and show the evidence!