Yes, you need both modules and the kernel.
However, nothing in the world is simpler than putting the new modules in, you just need to boot the device with ubiboot (just as you did when you untarred the FS originally there) and then copy the modules under the /lib/modules/<whatever_version>/ directory on your SailfishOS partition.

BTW, you do not need to flash ubiboot permanently to device, it it enough to use flasher to run it, just as you do when for example running maintanance kernel for malf-repairing.

Also, I just got near a computer, so when I get some time maybe today, I'll patch the CONFIG_CMDLINE_OVERRIDE option to my sources and recompile it (and modules) for you.