I have a client in a high rise that wants to upgrade, Currently there are two TV's one with an HDbaseT extender and another one with an HDMI over COAX extender. The TV with the HDbaseT extender only has cat5 run to it (building ran it during construction)

He wants to bypass the extenders for "optimum picture quality" The COAX one is easy since there is obviously coax available there. The other tv, not so much. Im reading conflicting information that the "clients" can get their signal via Cat5 or Coax.

It is confirmed that they are IP clients. Info is verry scarce out there on these as only Texas and parts of the Philadelphia suburbs are getting the new boxes. He is convinced the extenders are degrading his quality, even though he is right we cant detect it.

Any Thoughts?

this is using the Cablevision System Cloud DVR? not in my market but interesting -

Well, the client boxes will either work with the 1 cat5 or they won't. If they won't, what does he think he's going to do to get a signal there, besides tear out the walls to retrofit?

HDBaseT and the coax extenders do NOT degrade the signal quality any. It is bit for bit on those devices. Why does he think that it does? Have you explained to him they don't (I'm sure you have), what was his answer? Does he not believe you, the professional?
I'm assuming further at this point: that he has listened to some dumba$$ that doesn't know what they were talking about. Who was this genius and where does he hail from.

Going from memory but I think the client boxes were coax only on the ones we have done.

Why are extenders degrading quality?

I have installed the boxes at a few different locations, never tried the ethernet port though. Personally I think it's comical Verizon just sends these things out and expect clients to install them. The last one I did replaced a bunch of old boxes and support couldn't get the 2nd dvr to add to the account. It was such a fiasco and ended up taking hours upon hours, to the point where I had a Verizon tech come out to resolve the issue. I would expect nothing outside of the supported configuration to work reliably.

+1