now, this is an opinion, so....I'm not thrilled with the idea of new "fancy resonators"..the Yamaha horns are a fine design, and should work well with the factory set up..I'd perhaps try a bit of a stiffer reed. It's been my experience that focused long tone practice can produce a variety of positive results.

Making the resonators larger is the best chance you have at making a noticeable result. Don't fuss over plastic vs. metal - flat vs. domed, etc. Having said that, with all the expense that goes into outfitting a horn with new pads and resos, I think you would be disappointed in the outcome as the difference is rather subtle. If it's volume of sound you are after, going with a larger tip opening mouthpiece is probably the most direct route.