Bullet 2.47 is available.
Constraint solver has been optimized with a second path. Avoiding cross products during the iterations and gathering rigid body information into contiguous memory are the main changes. No huge benefits yet, but a good step in the right direction.
It can be disabled by removing 'SOLVER_CACHE_FRIENDLY' from
solver->setSolverMode(SOLVER_RANDMIZE_ORDER | SOLVER_CACHE_FRIENDLY)
Another stability fix is enabled, that adds some damping for rigid bodies with linear/angular velocity below a treshold. This heuristic is experimental, and can be disabled using
gJitterVelocityDampingFactor = 0.f;
No proper API for this, as it is experimental.