Yes and no. I do not think the lab experiments we've done have been the exact fusion reactions that turn hydrogen into nickel. But without exception every nuclear experiment has followed the tenants of special relativity, to the best of our abilities to measure --as much as ten significant digits! And there have millions of such experiments. And they jibe quite well with our observations of solar and stellar energy output... At least, as far as I know. Since the difference in mass between any nuclear isotope and the same number of protons as there are nucleons in that isotope is less than 1%, I feel very confident in my statement. IMHO, absolutely. And probably grav genorators that could both trap and extract energy from neutrons, neutrinos and other uncharged particles.... Which is just to say they'd be more efficient at extracting that <1%. And could also lead to artificial quantum singularities.