Given a curb weight of 2,700 lbs, the Mazda 3 with a 143 HP diesel would in theory do 0-60 mph in 9.5 seconds. However, the article claims 8.5 seconds, which if that would truly be the case, could be attributed to the engine's low end torque characeristics.
A common equation used to estimate 0-60 mph in gasoline fueled cars is:
(0-60 mph time in seconds) = (mass in kg) / (.9 * (horsepower))
It is relatively accurate to about 1/2 a second, except for extremely high HP numbers where sub 3 second 0-60 is possible and traction becomes an issue.
Using the above 0-60 claim in the article, we can make a sort of fudge factor to accomodate for the diesel's different characteristics. Multiply the right side of the above equation by .9.
The Miata weighs roughly 2,550 pounds. Plugging the numbers in for a hypothetical Miata with this engine would yield a 0-60 time of 8.1 seconds, on par with the stock Miata(~7.8 seconds for the stock MX5).
The mpg of the diesel Mazda 3 is claimed to be 55 mpg, but I assume that is imperial mpg. This would mean 46 US mpg. The stock gasoline Mazda 3 2L with 5 speed manual available in the US is rated at 31 mpg combined. So the diesel offers about a 50% improvement. I do believe the 2L is the same engine used by the Miata, only the Miata's is upgraded for a little more power.
Applying this to the hypothetical Miata, the gasoline version of which is rated at 27 mpg US, would in turn get roughly 40 mpg as a diesel.
Imagine the possibility, of a diesel Miata that does 0-60 mph in 8 seconds and 40 mpg. And we haven't even touched aerodynamics yet or did a weight reduction, used synthetic oil, installed LRR tires, or other obvious modifications.
That four-wheel double wishbone suspension on the Miata is to die for. Love it.