I'm currently trying with meat chicks on a diet of grains, legumes, soured milk, grass, alfalfa, oilseeds, and just a tiny bit of meat meal. Will let you know if they taste better... I'm expecting they will (since commercial chick feeds are fortified with artificial methionine, an amino acid vital for building muscles... Synthetic methionine is made from propane, among other ingredients). But this diet for chicks means care with cocci prevention.
We just ate two DP cockerels fed entirely the above diet. I'm still fiddling to get the feed recipe right in terms of growth, but meanwhile these two birds were the most flavoursome I've ever tasted.
The problem though is that cornish x are a very different bird. Whether they can develop flavour too remains to be seen. 
		
		
	
	
I remember my sister's lambs once got into her herb patch... The meat was amazing. Sage, thyme, mint...
If that all sounds too hard, try lots of plain old grass. It can definitely help.
E: Fixed typo.