I think you wouldn't be stressing them as much as you would be acclimating them to a higher alcohol level and attenuation. A 91 is going to take a lot to finish at the yeast's specified attenuation. The yeast will be going into the same environment they had just grown up through, otherwise I believe they would be truly stressed. This being said, the flavor may be indistinguishable between either and really may not matter. You may be more than ok with a low gravity starter, or you may be rousing your yeast later in fermentation to fully attenuate. That's my $.02.