Architect1776 wrote:
Your 100-400mm will not be heavy for the R7. It can handle the 800mm and 1,200mm lenses just fine so the 100-400mm MII would be no problem. The R mount also appears to be even more robust than the EF mount if that counts.
Obviously it would not be recommended to swing any large lens around by just holding the camera but if you were to just hold the camera the lens will not bend or break at the mount, it is not advisable though for many reasons.
Speed booster, I do not know.