increasing and consolidating standing rigging size

Jeff is spot on with his recommendations.

Increasing the 'strength' of the wire will not increase 'reliability' of the standing rig, unless you also strengthen all the chainplates, the chainplate 'knees', the wire to mast connections, etc. because the rigging should be tensioned to a uniform % of the wire breaking strength - usually to 12-15% tension. This is to precisely control the 'elasticity' of the wire.

If you do choose to increase the wire diameter, you will also have to have the luff of all your jibs, genoas recut to match the new natural 'sag' of the new thicker forestay when sailing. Specifically, if your new forestay (& backstay) is operating at less 'stretch' than the original 'design', you can very easily wind up with a boat that will have significantly less ability to 'point', and will be prone to 'skidding off to leeward' when attempting to point and with much more heeling -- a very 'cranky' boat when 'beating'.

If your intentions are to cross oceans, then perhaps increasing wire size would be prudent, including ALL the other components of the standing rig structure.

The issue that breaks 98% of all rigging is crystallization of the microscopic metal grain structure or 'fatigue'. Fatigue is cumulative and additive, starting from 'day one'. If your intention is 'serious' sailing, have no precise history of the rigging, replace it with new.

For the 'average' sailor (actually sailing, and not afraid to let the boat heel over) the typical recommendations for a total rigging change-out every 10-12 years is a good one .... about the equivalent of sailing one complete circumnavigation (about 1 million load 'cycles' where the rig, chainplates, and all the 'connectors', etc. 'possibly' have exceeded 30% of ultimate tensile strength).

FWIW - this would be equivalent to a 'blue water' design built with a structural 'safety factor' = 3 .... with respect to the rigging. Your boat is a 'coastal design' probably with a structural safety factor = 2. So, if this boat (@SF=2) has been sailed an equivalent of 16,000 nMi there's a high possibility that all the rigging is nearing the end of its typical 'service life' - and a higher probability of rigging fatigue failure.

Follow Jeff's advice, change the rigging, etc. to 'new' if youre planning on 'serious' sailing.

;-)

A riggers 'rule of thumb': 1 mm stretch per 2 meter length of wire is equivalent to 5% of the breaking load, irrespective of the diameter of the wire.
Ref.: p29 of: http://www.riggingandsails.com/pdf/selden-tuning.pdf

302/304 is more subject to chlorine/halide corrosion ... increases the 2-phase destruction: 1. accelerated crevice corrosion in the surface micro-cracks formed by 2. fatigue.
..... probably just like what happened to your OEM Tayana propshaft.

Im Ty37 #423 and I back-calculate a SF of 4! for the Ty37 rig ... BUT about 0.2 SF for the OEM Grand Deer toggles, etc. & have had 'surprise' failure w/ the toggle bolts when I first got this boat; and, about SF=2 if the chainplates arent routinely torqued. :-(

IMO - 316 or 316L is the best in 300 series stainless for 'ocean work'.