It is impossible to evaluate the contrast reduction caused by central obstruction by simply looking at a Strehl ratio. It is also impossible to evaluate the performance of any telescope with a refracting element (Schmidt corrector, Maksutov corrector, doublet or triplet) with the simple knowledge of the Stehl Ratio for one wavelength (usually green), which is sometimes given by telescope manufacturers.
Harold Suiter -
http://www.willbell.com/tm/tm5.htm developed another ratio (EER), which takes into account central obstructions.
Encircled Energy Ratio - The EER is the ratio between the total energy inside a given angular radius from a mirror's diffraction pattern to the total energy inside the errorless circular aperture's diffraction pattern.
The point I'm making is that the SR alone can't deliver the final verdict.