t court's certification order often bestows upon plaintiffs extraordinary leverage, and r*95] its bite should dictate the process that precedes it." Oscar Private Equity Ines. v. Allegiance Telecom, Inc., 487 F.3d 261, 267 (5th Cir. 2007), abrogated in other respects by Erica P. John Fund, Inc. v. Halliburton Co., 131 S.