For starters, it's easy to shift into classes/PrCs that would significantly boost their power. Even keeping a lot of the personal touches (Roy's ancestral sword, V's preference for blasting, Haley's archery, and so on), there are much better options, such as...
Roy: Warblade
Elan: Sublime Chord
V: Incantatrix
Durkon: Contemplative or (given the campaign) Sacred Exorcist
Haley: Dip into a casting class (e.g. Warlock) for spells that work well with archery
Belkar: Ye gods, just pick anything else.
Even keeping the same classes and sticking to Core, there's lots of little things that would make them more powerful. Buy/find Rings of Freedom of Movement, or have Durkon cast it. Build Durkon or V more around buffing/debuffing. Build Roy more like, well, Tarquin. Give Haley a Splitting bow. Etc.
I mean, it's plain that this would be bad for the narrative, but if you want to rebuild the OotS as a high-op party, you could start here.