Re: A little rigging advice please
Do you place more confidence in the engineer who designed your boat, or "the guys at the marina"?
As stated, upping wire size may require you use larger turnbuckles and clevis pins at the terminations. Then you'll be tempted to drill out clevis pin holes in tangs and chainplates instead of upsizing them too... NOW you could be stressing metal beyond its designed loads.
In short, keep with the wire sizes designed for the boat and intended for all the fittings.
That said... if you're planning use beyond the boat's designed intentions... say crossing oceans with lots of weather-related risk, then do up-size... everything, not just the wire.