It's like saying "I care about milk, not cows" . I'm talking about pixel color accuracy and fake resolution. Have a look at some Sigma DP2 Merrill RAW samples (ISO 100) and you'll see what's the difference between the real 15 megapixel resolution and the fake 18 or 22 megapixel resolution from 7D or 5D3.
Foveon is another area of great hyperbole in photography. An 18 MP Bayer sensor does not have "fake" resolution. There are 18 million sample points of luminance data. And a Foveon sensor does not have 3x its pixels in resolution. 15 MP Foveon has 15 million sample points of luminance data.
Foveon sensors do have more sample points of color data, and this results in better images then a mere count of MP would suggest. That can be seen and should not be denied. But it's not the night and day difference claimed by fans. Nor will they scale to match 30-40 MP Bayer images. This is the hyperbole. Go ahead and photograph a landscape with foliage using the Sigma and a D800e and see how the Sigma fares scaled to match.
With that out of the way, I wish Foveon would have had a chance with a larger and more aggressive company like Nikon or Canon. The technology is interesting. If it could have been pushed up in MP and ISO at a faster pace it might have had a much greater impact on the market. Again, it's not night and day. But it's certainly a strong edge in IQ.
I mentioned the Foveon X3 as a benchmark for Bayer sensor in terms of resolution (per-pixel color accuracy at low ISO).
I think your falling into the same trap as most when comparing a Foveon with a Bayer. Bayer is only limited relative to Foveon in terms of color fidelity.
A layered sensor design is capable of much greater color fidelity and accuracy because its capturing a full quantity of color information at every photosite. That also gives it another slight edge as it does not need a low-pass filter to eliminate color moire, since color moire doesn't exhibit. However Bayer sensors ARE detecting luminance data at every photosite, and there is no question they are capable of discerning a finer gradation of detail than a Foveon sensor...DESPITE the fact that their pixels are interpolated. I'm not sure an 18mp FF sensor is really going to be a significant edge, resolution wise, over a Foveon. But an 18mp APS-C sensor is going to resolve considerably more detail than a 15mp Foveon, and for that matter more detail than a 36.3mp sensor. Similarly, a 46.1mp FF sensor is going to be capable of the same resolving power as an 18mp APS-C.
The three-fold difference in luminance resolution and a far greater number of color pixels, several stops better ISO performance, and much greater spatial resolution, even when factoring in interpolation, gives a significant edge to Bayer in this case.