Juan,
'Is this what a matrix metering will typically do, to expose more for the shadows?'
I seldom use slides. But as far as I know, you expose for the HIGHLIGHTS with SLIDES. Expose for the SHADOWS with NEGATIVES. From your post, I think you exposed for the shadows when you used slides. I think that's why you got overexposed shots.
I don't find the NX to overexpose. I have used negatives mainly with it. I do one of these things for metering (aside from using an incident meter):
a) I would point my camera to the green grass or anything the would be close to 18% gray and click on the AE Lock.
b) Point the camera to the darkest area in the images with which I want to have detail and close down 1 stop. If it was measured at f/5.6 at 1/200, I would shoot it at f/8 at 1/200.
c) Other times, I would only check the difference between the spot and evaluative/matrix displayed on the viewfinder. If there is more than a stop or 1/2 stop difference, I would click the front dial the opposite way to compensate for the difference. This is my favorite feature.
d)With negatives, I set the ISO to 1/3 stop SLOWER than the actual speed to pop colors. For ex&le, if it's a 400 speed negative film, I set it to 320.
For slide film, you go the opposite way. You can set it as 50% FASTER than it's actual speed. For ISO 64, set it at 100. If the results are too dark for your taste, experiment with it. Set it at 1/3 or 1/2 stop FASTER to suit your taste.
Hope this helps...