Svelte
Your thinking of polybutylene also referred to as Quest plumbing. It was commonly used in the mid 80’s and something I warn clients about anytime we view homes of those vintages. A lot of realtors are unethical but just as many are new and dont know the history of various issues.A good test is to look at their license number. Mine starts with 013XXXX which means I was licensed in the early 2000’s. Im not sure what newly issued numbers are but if the license starts around 018XXX or higher your dealing with a newer agent.
EP
The answer to your question is maybe. There can be big differences between homes in the $1.5M range even with lots of comps. Lot size, view, upgrades, location within the tract etc all weigh heavily. If you have a premium product by those measures but base price upon average or below average homes in the tract you can leave money on the table. If the opposite is true you can overprice.
With that said there is room for incremental improvements and they have been occuring over the years via technology. However, homes will always be a heterogenous product and the transfer of them filled with legalities and liability. Some are comfortable with that others arent. Some will suceed going it a lone others wont. We all make our own choices and live with the outcome