1. As there are more disks in the VG, can them all work with larger
max_sectors_kb?

2. Does it work for md device without stacking LVM on top of that?

3. What's your Physical extent size?

vgs -ovg_extent_size vg_kvm # or simple vgs -v

4. If all above is fine then it may be related to LVM.

-- Marian