If the SD15 is anything like the current SD14, the pixel-dimensions of the produced JPEG is somewhere between 4-5 MP. But the RAW file should contain a lot more image-data - essentially nuances of color and light. This in turn may give noticeable freedom of options to tweak the image.
Personally I have no issue with that since - for most purposes - that is plenty for decent-sized prints.
If we by "sharpness" refer mostly to edge-definition, it's possible that the lower MP "suffers" a bit on account of the lower MP. On the other hand - when the image plays to the Foveon's strength - there is a lot of detail.
When I look at images @ 1920 X 1200 pixels (just over 2 MP) on my 24-inch monitor, I don't see anything noticeable in terms of sharpness. Mind you, I'm looking at the "good" pictures and I'm sure it might take a little more work and tweaking to make them. However, skin-tones and colorspaces exhibit a very attractive texture-detail and the colors themselves are just yummy to my eyes. And I'm pretty spoiled with my Nikon D40..lol.
I think that it's probably fair to say - judging from the SD14 - that a Foveon camera might not be a versatile as similar-priced CMOS cameras. Noise @ higher ISO and apparently white-balance in mixed lighting is a place where the Foveon requires a little more manual intervention it seems.
I wouldn't go as far as to say that it's a "special purpose" camera, but it almost is. When shooting to it's strengths I'm personally hard pressed to see anything that can match it - but that is of course a matter of taste.