I would modify the line spacing first and get that right. Then play around with the weight. For my versions of Charis SIL (other then the lighter ones) I used 15/12 for the weight. Some feel that's a bit dark and I've made one with about 1/2 the added weight using 8/5. I've not modified the bold and bold italic as their weight is fine with the others at 15/12.
While Blossom is happy with her weight, I find that a bit too heavy for me. But, at least it gives a choice. And soon there will be yet a third choice.
Today I am updating the readme file in the larger Charis SIL and releasing it out of beta.