Depending on the mount umask for /boot/efi, the wasted space could be used for a swap file. (Swap files should never be readable to users.) According to comments on https://askubuntu.com/questions/96522/using-a-fat32-partition-for-a-swapfile there is no filesystem overhead as swapon loads the blocklist of the swapfile into the kernel once and then the kernel accesses the blocks directly.