I to had the same concerns , I had 8ft Fisher HD plow installed on my 2011 F350 Lariat SD CCSB [no plow prep package] no problems at all with the exception that the front end drops a few inches when plow is lifted , I put it next to the same truck with plow prep and they were the same level with same plow.
I installed 2" leveling kit, air bags in front springs and rear bags , changed shocks to Bilstein 5100s . Rides awesome and you dont even know the plow is there ,
Very happy with this set up
PS My dealer installed the plow , so I really dont think the void warranty theory will hold water, Just my guess