There is allot to consider lost salary and cost of school, but I would think the earning power would be almost double assuming a specialty like radiologist, Oncologist or Anesthesiologist.
My brother just graduated from med school a couple of years ago, and I make more than he does right now as an entry level engineer. Earning power may be double, but the lost costs during med school and residency are not insignificant. The example below will illustrate the earnings over the next 10 years. It shows that at the end of ten years you will have made over $800k more dollars by going SRO, and total earning wouldn't be equal until 2/3 of the way through the 15th year.
Also, during the 4 year residency (and possibly during your 4 years of med school), quality of life will be better on the SRO side. 80hrs/week during residency are very common, almost normal. 90hrs/week is not uncommon.
It's very admirable if you have the passion and desire to become a doctor. But I think making that career decision based off of the money alone is ill advised.
Assumptions:
1) no pre-requisites needed (go straight into med school now)
2) cost of med school $40k/year for 4 years
3) radiology residency = 4yrs, paid at $53k/yr
4) median radiologist salary = $350k/yr
5) first 2 years of SRO spent in class at $85k/yr then up to $175k/yr after license
Year MD SRO
1 -40 85
2 -40 85
3 -40 175
4 -40 175
5 53 175
6 53 175
7 53 175
8 53 175
9 350 175
10 350 175
Total 752 1570
Delta = 818
Years until even = 4.67