I've played with jMockups demo and as a Balsamiq user I can say that one of its best features is "low fidelity", specifically lines that look hand drawn and comic sans font. That combo is what I consider most important in mockups: when you give that to a client, it's very clear that it's not a product, it's a mockup.
If you can make that kind of low fidelity, I think you would have very good mockup tool. Of course, seeing what you have right now, I can understand your thinking in pursuing high fidelity route.
I meant to say I agree with you saying doing one thing well might be a better choice for your product.
I liked what I saw in jMockups, I like how it works, and it's pretty fast. But I wouldn't use it as a tool to create mockups that I have to present to a client (that doesn't mean somebody else has the same kinds of clients or same needs as me), for the reasons it looks too real. Like I said, Balsamiq's greatest feature is that mockups look "hand drawn", what you called "low fidelity", one might call it almost childish.
You said jMockups and Balsamiq are less comparable than jMockups and Photoshop. I agree. So maybe you shouldn't pivot from focusing on high fidelity to low fidelity but excel in what you already do.
If you can make that kind of low fidelity, I think you would have very good mockup tool. Of course, seeing what you have right now, I can understand your thinking in pursuing high fidelity route.