Okay, let me be a little more clear. When I said "orbital speeds", I meant the speeds typical of an object free-falling into the sun. And I am interested only in an object impacting the sun, either at solar escape velocity or greater, or right up to relativistic velocities where the tau would create a significant mass increase. No decaying orbits, no gradual mergers.
Intuitively, it seems like a large enough mass moving at sufficient velocity would "burst" the sun, producing something like a toroidal splash effect in the sun's mass, and a complete cessation of fusion. Or maybe a smaller mass would create a shockwave through the sun that would create spontaneous fusion in layers that weren't fusing before. Maybe this would cause a nova-like explosion, or at least an immediate growth into red-giant size.
I'm just looking for what might be plausible, rather than eye-rolling, once you've accepted the premise of a mass coming straight in.