The endorsement will not come as a surprise to people who know Starr (at least the ones I know who know Starr), nor is it likely to have any real impact on her prospects for confirmation, which remain excellent given the extent to which Democrats control the Senate.
Although Starr was demonized for his investigation into the Clintons, he was and is a genteel man who, no doubt, believes in the old-fashioned rules for confirming judges and justices. Under those rules — the ones that Republicans applied to Ruth Bader Ginsburg but Democrats refused to apply to John Roberts and Samuel Alito — Sotomayor probably should be confirmed.
The decision for Republican Senators remains whether to apply the old rules. Starr still has some presitge, but probably not enough to help these Senators decide what the rules for confirming Justices should be.