Right, but that is compensated by the fact that that same time compression also applies to any energy released by the impactor. It'd be putting out >3.5*10^7 w

Right, but that is compensated by the fact that that same time compression also applies to any energy released by the impactor. It'd be putting out >3.5*10^7 watts / m^2 at 0.99c, from our perspective. And a single sand grain hitting it would release 27GJ. Not to mention... How on earth are you managing to keep something together when its being bombarded by hydrogen / helium at 0.99c?