Not sure how there could be considering what the subframe connector does. Hard to imaging obtaining the level of rigidity required for tying the unibody "frame" together with bolt on connectors. Maybe there is some company that offers something but I'd be a little skeptical.
Weld-ins are the same as bolt-in from many manufacturers. It's just the cost of hardware that makes them different. My instinct is the holes drilled through the frame rails weaken the frame more than weld-ins do.
Bolt in connectors are fine for most applications. Are they the best way to go, no, but they won't fail and rip out your frame rails unless you are running ridiculous amounts of power, in which case you wouldn't be running bolt in sub-cons anyway.