When you say 'just inside the toerail' then have a good look below deck - is the plate attached to the hull
or to a knee? If to the hull
, then moving it outboard seems pretty simple. Still, depending on how long the shroud is, moving the lower end 5 inches may add to the strength (wider shroud base/angle) but you must control how this changes the alignment of the fittings on the mast - you do not want any fittings to be exposed to bending, etc..
Adding another line to support the mast may make it stronger but you want the line to be balanced - the shrouds being laid out either exactly opposite (when in pair) or else in a 2x2 or 2+1 set-up. You only make things stronger/more stable if everything is balanced, otherwise resulting forces may bend the mast out of tune and compression can do the rest of the job. All this said, in a well designed rig, adding additional shrouds/stays is not necessary. For if the rig is not designed for the job then probably/maybe the hull isn't either?
If the wire and the fittings are sized for the job then going one size up with the wire will only add weight and remove some give, none of which may actually add to the overall strength and resilience of the rig.
There are some good books
so before you modify anything you may just step into the local marine
bookstore and get some. I always found the freebie at Selden pretty informative too (although probably too basic for the job at hand). Look it up anyway, maybe it will help you decide on the best solution.
BTW I found many local riggers willing to come along and give me free advice on my boat. Look around, if there is one somewhere close to your boat they may just cast a quick look and tell you whether you are right or wrong in your plans.