I've had the full-out custom job done on one of my Hi-Powers by Kurt Wickmann. He is an excellent gunsmith and does great work; but as a businessman, he can be hard to get a hold of and not real rushed to finish work by any particular date.
Having had experience with all the whiz-bang fun stuff you can do to a Hi-Power, I'd have to say that I favor Mr. Camp's approach more and more.
Not absolutely necessary in my opinion; but most people do it. It gives you drop-free mags and a little smoother trigger. However, you can have a good trigger without removing it.
Sad to say, but this is probably necessary on most new Hi-Powers
3) Basic reliability work...only if needed
I have it done on all mine; but I haven't noticed any difference in reliability between before and after pistols. 95% of the few malfunctions I've experienced with a Hi-Power (and I've easily run 5,000 rounds between malfunctions of any kind) have been magazine related. A few others were related to a worn recoil spring and I've had one malfunction related to an aftermarket barrel but it was a doozy
4) Single sided extended thumb safety
The stock ambi works well for me and even though I'm right handed, I like the ambi since I can confirm by touch that the safety hasn't been swept off (even though I've never had that happen).
5) Dehorn...only if needed
Some Hi-Powers can have some corners on them; but I think this step can be skipped for many Hi-Powers.
6) Tighten slide to frame fit and barrel lock up...only if needed
Just my opinion, but I think that this step is unnecessary. I have owned lots of Hi-Powers with a loose slide to frame fit and as long as the barrel and slide was tight, they shot quite well. On my custom gun, it rattled from a loose slide to frame fit. This was from right out of the box until I sent it off to Wickmann's. After the custom work, the rattle went away for about 2,000 rounds and then it came right back.
Good barrel lock-up is important; but tightening the slide-to-frame fit on a Hi-Power is like sweeping sand off the beach.
Depends on how the stock sights work for you. I don't like the fact that the dots on the stock sights extend all the way to the top of the sight. It clutters the sight picture for me. I also like tritium sights on my pistols.
I have Novaks on one of my Hi-Powers and I find them to be very fast and accurate sights. I would definitely use them again.
8) New grips (Spegal or equivalent)
Definitely a worthwhile addition. Most require some fitting though. So if you don't want to fit them, it is best to have the gunsmith install them when he does the other work.
9) Complete refinish of gun and mags with Robar NP3
I've got my Hi-Power finished in Roguard/NP3 and the NP3 has held up extremely well in every area but the actual internals where metal meets metal for every shot (and even there it is doing OK). It is an expensive finish and will probably add 5-6 weeks to your lead time; but it is a great finish.
Here is a detailed report on both the finish and the Hi-Power
http://www.ont.com/users/kolya/HiPo.htm