Yeah, it would be extremely surprising if we had anything on Earth that would do the trick (the layer of clouds in question is fairly nice, warm but not hot and

Yeah, it would be extremely surprising if we had anything on Earth that would do the trick (the layer of clouds in question is fairly nice, warm but not hot and fairly normal pressure, except for being chock full of sulphuric acid, so things like volcanic vents at the ocean floor or other typical "extreme" Earth environments wouldn't be a good analog for it), and even more surprising if it were able to contaminate a space probe as opposed to being confined to some, well, extreme environment on Earth. But with respect to time, I'm not sure that intuitively 40 years wouldn't be enough -- you're talking about an environment that is uniform, free from competition, and circulating rapidly (the wind circles Venus every 4 days). So the microbes would only need to adapt to ~one environment, not a gradient of conditions of varying temperatures, humidity, and illumination; any resources that were present would be up for grabs as opposed to already claimed by better-adapted organisms; and you wouldn't be spatially bound to slowly expand from a single spot at a snail's pace. It'd be one big petri dish under the right circumstances.