Air carrier Captain reported a wind shear and micro-burst encounter while tracking to intercept the localizer; resulting in various deviations.

Narrative

While being vectored for an ILS approach for runway 4L at DTW at 3000 feet; thunderstorms were in the area; the FO was pilot flying until I decided that I was going to take the landing due to the weather conditions; after we exchanged the controls we encountered severe windshear/or forming microburst. Our airspeed went from 200kts to 245kts in a few seconds; even though we were level and power was at idle; I engaged flight spoilers to try and control the massive speed increase. We over sped the flaps 8 speed by 15 knots for about 30 seconds and was not able to control airspeed (aircraft was written up to maintenance as required). Airspeed then decreased rapidly; I retracted the spoilers and starting increasing thrust to normal; airspeed continued to decrease and went about 20 kts below 200kts bugged speed; and we started sinking; I added thrust all the way up to max thrust to regain airspeed and altitude lost. When we increased to max thrust we were barely able to maintain the bugged speed. We deviated from assigned altitude by about 200-300 feet trying to recover from the downdraft. ATC issued a low altitude alert during this time as we were recovering. I exercised my PIC authority; and flew the aircraft counter to ATC's previous instructions; which was to turn and intercept the localizer; I choose to fly straight ahead until I could guarantee we had control of the airplane again. We informed ATC of our status at this point and requested headings to try the approach again. The approach from that point was uneventful. Cause: Unintentional encounter with downdraft or windshear associated with thunderstorms in the area. ATC advised us that rides were smooth with light chop in the area. The on board weather radar did not detect abnormally strong cells; light to moderate radar returns as we made the base turn. It should be noted that after we landed successfully; several other aircraft behind us had to go around for various reasons associated with the weather. Suggestions: This happened in a very unstable and windshear active atmosphere as mentioned by NOAA's Storm Prediction Center Tornado Watch that was in effect during this time. Even though the radar echos on the onboard Radar and even ground based Radar may have not been particularly impressive; mostly green/yellow with very little red; it is likely we penetrated one of these stronger cells and encountered the windshear/or forming microburst. Since these cells were small; the on board radar may have not been able to provide adequate warning of any of the stronger parts of the storm system. I think the suggestion here would be to always be particular concerned when flying in the area of thunderstorms and monitoring the aircraft's flight path and be ready to just fly the airplane as we did; fly out of the unstable weather and inform ATC of our intentions afterward.