I've only recently bought a zendrum myself but here's what I think anyway!
I think it depends of the style of music / the context / the aims of the drummer / playing style / the type of drums (acoustic vs electronic samples) / the needs of the actual piece (kick/ snare/ cymbal etc).
I guess you could also separate expressiveness
(p,mf,ff etc) with realism
(not hearing the exact same sample repeated ie machine gun effect). Usually the two go hand in hand ... as you increase the velocity layers the dynamic resolution increases and this means more chance of selecting different samples for similar velocity hits.
But another way to build sample sets is to program them with a 'round robin' feature so that even if you had only four velocity layers, for each velocity layer there is a bank of 5 pretty much identical samples which would alternate in sequence. This method might be preferable if you want a small sample set and you're going to be playing within a very narrow band of the dynamic range - ie whacking the crap out of your zendrum at velocity 127 all the way! You can do round robin programs in Kontakt BTW!
You asked about the sensitivity of the zendrum - this is what I think:I think the pads on a zendrum offer as much sensitivity, and therefore expressive potential and realism, as any sample set could possibly hope to utilize. IOW any limitations will be down to your sample sets not the zendrum - or to put it another way, you can't ever have more samples than the zen can make use of.
If your aim is to play acoustic drums and make them sound as real as possible then strictly speaking
I'd say you need a lot more than 4 layers! (but then again it's all about context and personal preference!). I use BFD and the original sample sets had between 20 to 45 velocity layers for a snare drum - at the time this sounded amazing and very expressive and realistic ..... but then when new packs came out for BFD with 70 layers .... and then 200+ layers things sounded even better and the old 46 layer snares started to sound a lot less expressive and realistic by comparison. However that is when playing the drum sets with a zendrum like you would play a real kit at a regular drum practice ... but if you used those snares in a heavily compressed mix in a song where the drums are playing a simple punky / rock rhythm and competing with guitars/ bass etc then you're probably not going to hear those differences so much or appreciate the realistic nuances that 200+ layers can give you!
Personally I find ALL drum sample sets to be a huge
compromise on the real thing and could never regard them as even the 'same instrument' as my actual real acoustic kit! - I mean whether playing zendrum or v-drums I have to play a different style to accomodate the limitations of samples especially on things like ride cymbals etc - so for me I think zendrumming only really became appealing once these mega velocity layer kits in BFD appeared and I could at least get pretty decent flams / rolls / ghost notes etc as well as never hear that dreaded machine gun effect!
But having said that, when playing electronic sample sets or more whacky ('industrial' or whatever) style acoustic kits, I can be totally happy
with even one
velocity layer! .... it's only when emulating the expressiveness / realism of 'real acoustic kits' where I get so fussy!
So there's my two cents! - I don't want to sound like a 'sample layer snob' - I can't stress enough how much the context has a bearing on whether any particular sample set works or not. I think 'realism' is one thing but 'sounds great' can be something else entirely. You asked what's practical
- I guess that depends on your set up and the music you want to make, but I guess the point of all my rambling (sorry!) is that IMHO there is not really an 'upper limit' where adding velocity layers won't improve the realism and expressiveness of the drum ... certainly not before 300 layers! LOL!
All I would say this: relative to the cost of a zendrum BFD2 is really not that
expensive ..... !