The first question I have after a quick google search, Is it a 285 or 285M? The 285 has a plastic spool and magnets wont do a thing for it. The 285M has a chrome plated, stainless steel spool. Adding magnets only works with metal spools (material does not need to be magnetic).
This model reel was not designed for casting, but rather a boat reel for bottom fishing. The spool material itself, if metal is going to cause problems when trying to cast. The spool is very heavy and requires a large amount of energy to get spinning up to speed on the cast, Then just as the spool is spinning fast enough, the weight and bait will have lost a lot of inertia and be slowing down already. The same weight that took so much energy to get moving, now requires a lot of energy to slow to keep pace with the slowing of the outgoing line. This is where adding mags may help, how much and if in a significant enough manner could only be determined through trial and error.
In the long run, you may find that this particular reel will never be more than a mediocre caster.