Not really. They seemed to have gone for the twin logics that the girls should be new, inexperienced, and fresh to match the book, and that they couldn't have been on TV for the lets attack reality TV angle. As soon as they decided that they seem to have cut out casting any of the really strong 20 somethings ALW had sucessfully found for his Dorothy series who now have experience, and anyone else outstanding who had established themselves recently in the young roles in WWRY, Mamma Mia, Blood Brothers, Legally or Wizard.
The problem with the Spice Girls isn't that they can't sing.They obviously worked well enough together to sell. It may be that not all of them sang much and one ends up singing not much at all. That means you can't recreate a group of 5 without it sounding different - or people noting a changed character, and having 4 identifiable singers begs the question who has gone. That may mean you then revert to the show not being about a band really at all, and certainly not one with the familiar strong characters or similar ones. Once you do that, you end up not recreating what the Spice Girls do well which is sing together, in combinations that sound striking and/or good, whilst providing striking individual characters. Mel B and Mel C closing Viva Forever, and Emma leading on 2 Become One are beautiful - if done properly. its difficult to see how anything works if you don't have similar voices singing it in very similar ways with similar harmonies. If you don't have anything that actually sounds like the Spice Girls, you probably need stronger singers than the Spice Girls o make it sound good.
Either way - going for a Spice Girls sound or great solos and duets - there's enough potential there for enough great songs. If you created a West End all stars group increasing the age range up to people who could play late 20s, or cast a group of TV Dorothies and other recent young successes, I suspect the Spice girls songs could sound really good. Actually, if you just used some of the current and recent WWRY Meats and Scaramouches and their understudies, I think it might sound brilliant.