I’ve been experimenting more with this stuff too recently. Here’s where I’m at:
I think sighter leader is most valuable in a high stick/tight line type of situation where you’re not actually fishing with any fly line, just all leader. In that situation you want to be in as much contact with your nymphs as possible, usually with the nymphs rolling the stream bottom. A traditional floating type of indicator will interfere with that and may cause the nymphs to suspend to some degree too. You’re largely just fishing by feel here anyway, but by using sighter line at the section of the leader that meets the water you can gain a visual cue of a strike as well, without losing the feel/contact with the nymphs like you would if you had a floating indicator on.
I don’t think having a foot or two of sighter leader further up in the leader would affect dry fly fishing that much…If you were nymphing with that set up odds are you’d have several feet of normal clear tippet below the sighter section as is…just tie your dry on to that. No need to swap out the leader in most situations IMO. Unless you’re trying to fish dries in glass calm water I don’t think that short stretch of sighter leader further up will interfere with anything when switching over to dries. And if you’re fishing glass flat water, odds are you weren’t just tight line nymphing it moments before.
Some guys use tippet rings or other fancy gimmicks to tie in their sighter section to make it easier to swap out later. I just tie a section of the sighter in about where I think it will be at the surface (+/- a foot or so on either end) as I’m stepping down my leader toward the end tippet. The sighter material I have is about 2x I think, as that's about the section of leader most frequently where I'd need it at when nymphing. I'll then step down from there to 4x-6x depending on the application.
Corollary - For longer casts where you need fly line to make the cast, and then fly line will be on the water to some degree as you’re fishing, disregard the above, better to just stick with a floating indicator. Which type of floating indicator you go with is personal preference. I like the little bobber guys, FWIW.