"Of course we could do that experiment with Venus...Would it thin the atmosphere or heat and expand it? Put on your thinking cap and imagine the before and after comparison possibilities."

I would say it would be hard to predict but it would also depend on what hit it: how fast and how big and whether it hit at low angle or not. I think anything less than a 1/2 km in diameter and with velocity of less 30 km/sec
wouldn't have much of an effect- I'm not even sure it could even hit the ground- unless it was at a steep impact angle. If small rock didn't hit the ground and throw up dust the added heat would only expand the atmosphere in terms of days to weeks, I would imagine.
Also it is a given that Venus has been impacted "recently in geological terms" by rocks which would be larger than 1 km (I also wonder if what see in present Venus is largely the result of a "recent" impactor- also the lack of impact craters may indicate that few impactors have penerated the thick atmosphere rather than resurfacing due to global volcanic events).