The resolution of the lens used in the Crop sensor needed to have 1.5 time more resolution than the one used in the FF to give us the same sharpness.
This is false. Resolution and detail contrast (sharpness) are two related but separate things. The measured resolution difference (in lpmm) between FF and APS-C sensors of comparable pixel counts is small as good lenses still comfortably out resolve sensors at MTF10. Adding more pixels to the APS-C sensor would not alter the lens MTF curve. The loss of detail contrast happens in the lens, not on the sensor. (Assuming AA filters of equal strength.)
Read this :
I would rather watch paint dry then read DxO theories.
For the pixel peepers, it's rather worrying the amount of resolution lost in cropped systems.
DPReview measured the 5D2's resolution to 2800/3300 (absolute/extinction). The 7D was measured to 2500/3100. The difference is smaller then the difference in sensor pixel count. (So much for DxO theories.)
There is very little to gain by sticking an L lens on a crop body.
Apparently you've never actually tried this. FYI, I can see the difference out of camera between the 70-200 f/4L and 70-200 f/4L IS on a 7D. The difference between either L lens and a consumer zoom on the same body is quite large. (The L vs L differences probably wouldn't survive post processing, but the L vs non-L differences definitely would.)
For example you won't gain 8MP in overall resolution by jumping from a 12MP 450D to a 20MP 70D (crop of course).
You will gain exactly 8 MP because MP is a measure of the number of photo sites on the sensor. The fact that DxO plays fast and loose with definitions and creates meaningless ones like "perceptual megapixels" is the first indication that they are a pseudo-science site.
Your percentage gain in image lpmm will not be equal to the gain in MP because there are losses. But the losses are nearly identical for a 12-20 MP jump in the 35mm format.
Can we possibly say this :
Lets assume a 20MP crop canon here....
20/1.6 = 12.5
12.5 being the maximum MP you could possibly hope to get.
No we can't say that. Strictly speaking that is absurd because MP always refers to the number of photo sites on the sensor. I know what you are trying to say, but we can't say it even with correct terminology because all observations are to the contrary (see below).
Answer : Yes it is. Bigger sensors capture more detail even when compared to a crop with the same MP. (6D vs 70D = no contest, 6D wins)
Detail != sharpness.
For sharpness, I agree that a 6D will be sharper out of camera all other factors being equal. And I'll add that the difference will not survive post processing unless the images were made above ISO 800.
For detail, Imaging Resource has measured and published the resolutions of both, and they are nearly identical. 2400/3400 for the 6D and 2500/3200 for the 70D.