I have an ND filter which clips into the SX-70 lens bezel, rather than over the film gate on the pack. It cost me about $15 for an all glass, multicoated filter.
I've only shot a couple of packs of the IP 600 film (two 600 series Polaroids were given to me last Fall). My impression is that it is, like the Spectra packs, an easier emulsion for them to produce and a bit more consistent than the SX-70 films have been.
With the SX-70, due to the fact that their nominal rating for the films has been ISO 100 and the cameras were designed for a very consistent ISO 80 film, there's always a bit of inconsistency due to just how you set the Lighten/Darken control, which itself seems to be a little variable camera to camera ... at least amongst my three SX-70s.
G