grogB

A speakers frequency response will change with SPL, at the lower frequencies the effect of the suspension stiffening and the BL dropping due to the coil leaving the center of the gap will increase the Qt and decrease the sensitivity, also the coil impedance will rise dur to thermal effects. there is lots of data on this from klippel the high frequency response can also change with level, I've seen prototypes where the high frequency roll of of a 6" driver changes from 4Khz at 1w(86dB) to 1.5Khz at 80w(102dB)

apart from the improved mechanical rigidity the cast aluminium frame will give more motor strength. A steel frame will draw some of the magnetic energy away from the magnet system. the effect is not huge but certainly measurable i'd always go for a cast aluminium frame if i had the choice.