It's a legitimate concern.
From my non-Zeitgeist experience, I've found that Pathfinder starts to break down after level 12, and completely falls apart starting at 16. Not so much because of math, but because of the bloat of options, and trying to handle all the effects and creatures simultaneously. (Want to kill all fun at a table? Cast
antimagic field in the midst of the party. It's only a 6th level spell (thus avail at 11th level), and then everyone has to recalc their entire sheet, as
so much of a character's (particularly melee warrior's) stats come from gear.)
Anyway, my hands-on Zeitgeist experience only goes up to 6th level (which might not sound like much, but ZG is a huge campaign with a slow curve: that's 4 adventure books and 9 months of play). But from what I've GMed firsthand (and what continues to be validated by reading ahead) is that Zeitgeist really, really is different. The clearest case-study in this is the battle on the Coaltongue, the 2nd fight of the campaign. If you haven't already: study it. Not just read it, but really
study it. Look at the players' goals, the constraints the game sets up, and the various strategies the players would want to take. It's not at all a "surround them all and kill them" sort of affair that 99% of combats in a Paizo/WotC module serve up. And it's difficult to understate how profoundly this effects tableplay. I will go as far as to say it's like you're playing a different game altogether, and is one of the reasons why I'm a huge ZG fanboy. (Incidentally, it also likewise effects character builds, as the typical "go for huge DPS" is no longer a go-to melee strategy.)
Now, there's a downside to this too. Combats in Zeitgeist take longer. The PCs are spending time doing things other than systematically whacking away at their foes, and there's usually a lot of terrain or strategic considerations. For every fight. A mitigation on this is that there is virtually no "filler" combats. Pretty much every combat is either foreshadowed, against a named NPC you've met before, or advances the plotline in a decisive way. (Again, this is very different from the Paizo/WotC implementation strategy, which tends to have a lot of extra monsters lying around just to eat up attrition.) So it doesn't feel as much like a waste to sit through the (sometimes quite long) combats.
However, I should mention one "overhead" problem that Zeitgeist has: the stipend system. Basically, Zeitgeist has very little in the way of planted/lootable treasure. (But that which does exist is always colorful, plot-based, and worth holding on to. But they'll never cover your "big 6", for example.) The game instead gives you a stipend (basically, your in-game boss pays you a salary) and more-or-less every time you level up, you get additional stipend to go with it, to keep you pretty close to WBL. At very low level, my players loved this, as they got to pick exactly what gear they wanted. But it
quickly overstayed its welcome, and became more of a chore than a treat. ("Dammit guys, we gotta spend 10k
more GP before we can start the session.") The higher level you get, the more purchase-options there are, and the more stipend-gold you have to throw around. Also, as you get higher and higher level, it starts to beg the question of "would that really be for sale?", if a player asks for something peculiar, perhaps a
+3 rifle of speed. With a
+1 vorpal bayonet. With
energy drain. And armor-piercing bullets. Filled with witch-oil. (Which is a reasonable ask at high level - but then you're constantly adjudicating what is "in stock", versus what you can "special order" (and on what timeframe), versus what might be illegal (and thus available with the right contacts), and it quickly grows tiresome.) It's just too much, too often. One great thing about 5e is that it is far less dependent on gear for power. For PF, it's arguably more essential than level.
One option: play ZG under the PF rules until "enough" of it has come out in 5e, and then switch over. You could just do this at an arbitrary time with no in-game justification, or...
[sblock]The climax of book 9 involves breaking the laws of reality itself. Starting in adventure 10, magic and physics itself is reforming. If you needed an in-game justification to change rules systems, you couldn't have asked for a better excuse. FWIW, this happens when you hit (PF) level 17, so it might be a bit late if you're trying to avoid the teen-levels altogether.[/sblock]
Finally, one cold, sobering thought: you're not actually likely to finish the campaign. It's a
long campaign. Thirteen books!!! Compare to the Paizo Adventure Path system that only does 6 books. This is literally more than twice as long. (ZG levels you up much slower than Paizo modules.) And it really is more content too. Each ZG book is anywhere from 80-100 pages, all of which is game-content. Each Paizo AP issue is 96 pages, of which usually 10 pages are not game-content (the fiction, and there's always extra monsters in the bestiary). You're signing up for very, very, very long campaign with ZG. So, all the concern about high level PF might be moot.
Just being practical.
Anyway, good luck!
So I'll have to try to talk my gaming group into sticking with Pathfinder. I think I can handle that part. (There's no chance at all of 4e.)
