[Aside to everybody else: Apologies for the lengthy reply. Am I missing a way to reply directly to Diane?]
But I’m wondering how much I am losing by going over wifi.
Potentially, a lot. It depends on the hardware on both ends and conditions between them. However, there are other limitations in your setup.
The answer to the question in your subject is the slowest link/node in the chain limits the end-to-end speed.
Your modem may be dragging down your cable speed. In the product description on their website, NETGEAR says the CMD31T is rated for “Up to 150 Mbps†” (“† Maximum rate is derived from DOCSIS® 3.0 specifications and actual throughput will vary depending on the MSO provisioning and speed tier subscription.”)
The connection to your AirPort Express may be dragging your speed down further. A refurb AirPort Express bought in 2016 is likely to be the 2012 (last) model, so it can do real 802.11n on the wifi side, but it is limited to 10/100BASE-T Ethernet on the hardwire side (both WAN and LAN ports), dropping you further to 100 Mbps theoretical maximum of your 250.
Your 2015 MBP Retina should be capable of 802.11ac and compatible with 802.11n, so that hardware shouldn’t be a limiting factor.
The floor between the AirPort Express and your MBP will also have an effect on the speed achievable, even with good signal strength, depending on materials and thickness.
You can see what protocol and speed your AirPort Express and MBP are negotiating by holding down the option key while pulling down the wifi icon in your menu bar. (You may need to enable that icon in System Preferences > Network > Wi-Fi.) Check “Tx Rate” and “PHY Mode”; you would like to see something near 100Mbps and 802.11n respectively.
While you have it pulled down, you might check “CHANNEL” to see if you are using the 2.4 or 5 GHz band. 5 GHz can offer higher speeds, but it degrades more with distance and materials in the signal path.
Best of luck resolving your speed issues.