if all the parts are the same material. sure.
you'll have some extra pre-processing time setting up the problem (create the part, merge the bodies, mating it..etc)
will it reduce computation time? if everything is touching face to face and you're using compatible meshing, probably not a lot. maybe a little bit. you might be able to get away with a larger element size for example. if you're using bonding contact sets, probably get a bit more speed out of it, but again, not a lot.
the beauty of the automatic bonding is that it handles the case you describe for you. if you had to create contact sets like other software, then i would say go with the method you're suggesting.
Thank you Jared!
Luckily I came to check for a response before I started modeling the sub-assembly as a part! Saved me a fair amount of time, and thank you for the detailed response! It helped me understand the answer rather than just being told "yes" or "no".