I recently saw a feature shot with a combo of Super-8 and S16 as a blown up print and I have to say (though I was in the front row) - it crushed any ideas I'd ever had about shooting on Super-8 to blow up -- it looked like old Video-8 as a blow up NOT Super-8 -- the blacks had all kinds of colour grain artifacts -- which is likely to happen even if it's Tri-X unless they're going for B+W print stock? Sadly, the blow just lost all the grooviness and that beautiful feeling only Super-8 has as projected 8mm print .. sorry to say -- I'd go a different route altogether to achieve this look. Not to mention the HELL your post team will go through syncing sound (you'll most likely require front and tail slates for every take in order to post sync the dialogue) Arrrgh! And undoubtedly, the post team will say, "Who shot this piece of sh$#? The camera speed is all over the place"
PS. Please don't shoot the messenger!