I think it would be prudent for 818 to send a query to Amidon for an opinion on the HP cut. I think one is as cosmetic as front serrations but that's only my opinion.
I didn't see the trough you mention. Were you referring to 818's pics? I only saw a flat top which is allowed (as are tri-tops) and normal serrations. I've seen an actual trough milled into the top of a slide using a ball end mill on limited guns.
Regarding the trough, I was thinking of tpcdvc's pics.
"Milling of the slide to insert sights, add or remove serrations, such as cocking or flat topping, tri-topping the slide, lower- ing ejection ports, cuts that are minor and cosmetic in nature are permitted. Duplicating features that are on a factory, mass produced slide available to the gen- eral public is permitted. Cuts that are designed to specifically or significantly lighten the slide, such as holes, or slots, are ruled as competitive advantage and prohibited."
The ball cuts or "trough cuts" are only .015 deep and did not even register a difference on my postal scale before and after. I think that would be the very definition of "minor and cosmetic". I guess an ignorant RO could try to bump you to open, but that is an arbitration that I would pay for in a second.