I might catch some flack here, but practically anything would be an upgrade over the stock powertrain. The question is whether the swap is worth the effort.
Ya, this subject has been argued many times here. Personally I completely agree with your comment. Another option I am exploring is a conservative turbo setup on the X's 1500 engine. My intent at the time I started it was for the money and effort it would add enough power to make the X more liveable, without the major job and expense of a swap or even a performance NA SOHC build. So far I'm into it for a lot less than imagined. We'll see what happens.
"find an "engine donor quality" 2002-2011 Civic SI", "For $3000 or less, you can have a complete car.", "A3 dropouts go for $1000-1500 and Z3 dropouts go for $1500-2000."
When I looked into the prices of donor cars a couple years ago there were several great choices with "B" engines for around $1000-1200 (asking prices) locally. That is a complete car with just about all of the components you need. More on cost....
"That's not exactly fair, though. You're comparing a full DIY project with a kit-based project. The kit makes the swap vastly easier, but if you can B swap an X without a kit, you can K swap an X without a kit."
That's not fair either. First, the B swap is much easier to do because it requires almost no cutting of the X compared to the K. Second, the B can fit with a simpler install "kit" (DIY or otherwise), which would make that less expensive either way. Third, one might be able to do the B swap but not the K swap considering the level of modifications needed to do the K vs the B.
At the time I saw the B cars for around $1000, I also priced the whole swap project based on what everyone was reporting on the forum.
IF you kept it basic (no turbo, no custom wire harness or ECU, etc) and did it with all of the stock components found on the complete donor car, then the only real expense would be the axles. That's assuming a DIY as you say. The total estimated budget for EVERYTHING on a B swap was far less than any donor K car (including the $3000 you mentioned). Realizing that isn't an extremely high performance swap, it is still several time greater than any X engine. I posted a detailed rundown of this somewhere here.
Following
@lookforjoe comment, the Chevy EcoTec has also been done. The one I haven't seen yet is the Mazda/Ford drivetrain we discussed previously.