"What would the "more sophisticated proprietary data feed" contain that the public feed wouldn't?" The SIP is often stale. In some products, it doesn't matter. In many of the most active products, it's well behind and frequently of little value. For some products the SIP only sample quotes - especially size.
I presume when you say SIP you are talking about the machinery that combines all the info from all the different venues into one stream? If so, I am not sure how the info could be stale, unless the SIP doesn't keep up with the data flow? And if not SIP, would the "more sophisticated proprietary data feed" give you direct feeds from those venues and let you combine it yourself?
Think of the SIP as the top of book consolidated tape. Private feeds are just that and they are quite costly depending on the venue. SIP can be redistributed at a per terminal cost. Private feeds have very limited redistribution provisions. You can see prices on the venue sites. Not certain of all the details.
Facts about the Securities Information Processor (SIP) - https://www.ctaplan.com/index I'm not sure what the SEC is asking for or what the result will be. It is my hope that the focus is on providing access to live quotes at a reasonable cost to promote more participates to the market. IMO, non-pro fees are very reasonable. What is not fair, IMO, is the treatment of a non-natural person being held to higher fees. A single member LLC is treated the same as a Hedge fund. Non-display fees make it very difficult for a retail customer to have automated access to the markets with live data. When the SIP feeds have a monopoly, the SEC needs to step in and mandate exceptions for retail customers that want to participate with their small companies.
WONDER what the other exchanges position was? THAT article only noted 3 big ones . Like, another exchange opinion , like the the one that put 30 miles of wire/speed bump in its building/buildings ??[Prop data maybe a bit faster+ in some case$ may not include 30 miles of wire/speed bump in same building]