My rounds feed smooth as silk. So I doubt very much there are any bends in the tube. Both FN and Mossberg sell shotguns with 8 round extensions and no support. So it would seem that the worry of bending the tube may just be marketing hype to sell those barrel clamps.
Mossberg 930
FN SLP Mark 1
Like I said, if the 870 Mark 1 barrel clamp was available, I'd grab it. What I see so far in the form of barrel clamps is just a waste of money.