I have a m52TU 528i aswell. I converted to the m50 manifold along with catless headers, s52 injectors, cold air intake and a few other goodies. Definitely woke the car up
DISA is already mapped to be open all the way, so no more gain there. Next upgrades would be M54B30 intake camshaft and intake manifold, that would give extra hp in the top end.