I put a byrd shellix in my 15" planer last year and did a fair amount of research on spiral vs helix. Byrd is the only after marker helical that I found. It may be the only one out there period. PM offers a helical but uses byrds. Jet claims a helical but doesn't say who's it is - I bet it's byrd, too. From all the reviews, helicals are a little better than regular spirals on extremely figured wood than spirals but for most "run of the mill" figure, they are about the same. The shelix slices but leaves a slightly cupped surface which some people don't like though I have no complaints after running lots of Jatoba though my planer. Jatoba is notoriously hard to sand so it's close to a worst case scenario - looked fine to me though. And, a $10 cabinet scraper works really well on it.
To the question of Griz (non-helical) spirals vs the byrd shelix - no, they aren't marginal. For the vast majority of wood, you wouldn't really see any difference. But you WILL see and hear a big difference with either over a straight blade head. If I had a choice of shelix vs spiral, I'd take the shelix but not at much of a premium. I'm looking at a new jointer and would happily take a spiral instead of a shelix.
That cu tech "spiral" looks empty. Not enough cuts per second. One of the reasons the byrd or griz spirals work so well is they have a lot of cutters and thus take smaller cuts. The cu tech head has 24 cutters while the byrd drop in for the DW735 has 40. At this point, the math is optional.
By the way, griz sells the byrd shelixs and some times has them on sale. I think I paid 580 for my 15" but they are going for 655 now.
Final note: upgrading a planer head is a fair amount of work. A skilled mechanic will have no problem. I consider myself semi-skilled and it was a challenge.