What size treble hook replacement should I buy for my jackal ganterel jr swimbait. I am thinking about getting some owner st's but what size? 2/0 or 3/0?

For sure not anything that big, those would be too big for a full size Ganterel. It's size #2 for the Jr is what their site says.

I read an article (somewhere) the author downsized hooks on hard swimbaits and had a better hookup ratio.

Rule of thumb is to go with the same size as the manufacture did or up it one size only.