My take on it is that it's harder to find a reflector-only setup that'll control light spill well enough to avoid pissing off oncoming motorists with glare.
Is it possible
, particularly with good care towards aiming?
Yes, I do think so - but it'll depend highly on the specific vehicle/headlamp assembly.
With respect to your desire to buy aftermarket projector headlamps, peterd
, don't get too hung-up. If you can find a set of aftermarket projector housings on eBay or the like, on-the-cheap, just do it.
Instead, devote your funds towards obtaining, after you have the housing in-hand, good OEM take-off HID components so that you can pursue a full-out full-optics retrofit.
Because in many, many cases, aftermarket projectors simply suck at light output and throw.
I was a long-time DSMer before finally going with a 4-door vehicle, with my '05 Legacy.
My first DSM - way back when I was in high-school and first got my license - was a 1Gb 5MT Eclipse GS DOHC, new-off-the-lot in '92, when my parents spoiled me and bought it for me as a gift. It started an addiction that was to last until my current vehicle, also bought new in '05.
What does that have to do with anything?
Well, given the community and the timing, I saw both the availability of aftermarket projector headlamps as well as aftermarket plug-and-play HID kits come to-market, over the course of those two decades or so. And the two things that we learned, as a community, was that sometimes (more often than not, actually), aftermarket "dress up" projectors truly offer disappointing - if not outright dangerously low or mis-shaped - lighting output, and that in many cases, in order for there to be adequate lighting and light control, with use of the plug-and-play kits, we'd have to find ways of altering the headlight's optics: i.e. dropping in reflector bowls, lenses, cut-offs, many times as entire assemblies, into the provided projector housings, and then adapting everything to work together - aka "retrofitting."