What are the tradeoffs between spiral and C-shaped dough hooks?
The C, J or spade hook is a dismal excuse, having an older "professional" (as you say, in name only) 5-quart Kitchen-Aid mixer that came with one. Per comment above one evidently cannot just swap to the spiral hook on that era of mixer due to the bearings being different. The C hook works against the sides of the bowl, while the spiral works against the bottom, so it pushes up on the shaft.

Of course, I'm comparing against the way that a 30 quart real commercial unit works, with its spiral hook. I wanted a mixer for my kitchen that worked just the same, but smaller, and it's not...

So while you may find the faux-commercial spiral disappointing, you may not have plumbed the depths of the well of disappointment; unless you really want to go there, stick with the spiral, bad as it may be, it's probably better than the C.