As part of the work in our kitchen & dining room, the builders jacked
up the joists under the dining room across the middle so it's a lot
closer to flat & no longer bouncy, then covered the floor of the
combined rooms with 6 mm plywood (using a lot of screws).

We are planning to get an LVT floor put down, & the flooring company
recommends adding a layer of 6 mm SP101 plywood before gluing the LVT
down. There's plenty of room for the extra thickness, because we had
originally planned on an engineered wood floor but it's impractical to
make the two rooms flat enough for that.

They don't require the SP101 layer, but they will give a better
guarantee on the flooring with it. Any comments on this?