If you find yourself at an open house with a chance to chat with the agent, best to keep the questions to facts about the property. You will not get anywhere challenging the agent about price (even if it is seriously aspirational!) Two things to remember; the agent probably had a hand in determining the listing price. The agent will want to defend said listing price (at least for the first 30 days). Then at that point they may have to have that difficult conversation with the seller, the one about the price being too high. Many listing agents will pitch to a seller a price that is a aggressive though in this market anything is possible. And it is only human nature to list with the agent telling you he will get you that price.
I always felt that the market is more persuasive than just about any golden tongued sales person. Price a bit soft and let the market do it's thing, less time on the web and probably will attract multiple bids after the first open house. Sort of like investing in index funds, 90%+ of the time they will beat the active manager.
Back on topic. You corner the listing agent at an open house or private showing and hit him with data about the sale 2 floors above, 6 months ago that was 15% below the current ask of his listing; I can assure you the conversation won't go well. If the property was just listed your comments may just be brushed aside like an annoying mosquito. However if the home has been listed for 15+ days the agent may get a tad bit annoyed or perhaps unload a bomb on you! This is not a good way to begin a negotiation.
I'll wrap it up here. In short there is no advantage to telling the agent his property is 15% over valued.We have ways of presenting offers that are significantly below ask, we call it a "soft offer". Certainly there are times we feel our clients are off the mark with a valuation. In those cases we talk it through to try and understand each others position.
Sometimes the best thing to do is just get the conversation started, at least then there is the potential for success. If there is no discussion there is little hope for success.