The Nest WiFi Point units have very similar WiFi specifications to what Google WiFi units have (AC1200 2x2). The Nest WiFi Router units do have more capable WiFi radios (AC2200 4x4), but if you added one of those to an existing Google WiFi system, the performance improvement would be limited by having to talk to the Google WiFi using two streams, although it would be able to talk to clients using more than two streams. But, many clients don't support more than 1 or 2 streams anyway, so whether that would actually be faster might be moot.
An alternative to adding a Nest WiFi Router as a mesh secondary to a Google WiFi system would be to create a new network (with the same SSID+password) using a new Nest WiFi Router unit, and then (after doing a factory reset) add your existing Google WiFi units as mesh secondaries to that. However, similar limitations would apply – only clients with WiFi radios that support more than two streams would see improved performance, and only when connected directly to the Nest WiFi Router. So, whether this would actually be faster or not is hard to predict.
The biggest performance boost you can get would be by running enough Ethernet cables to get your existing Google WiFi mesh secondary units connected back to the primary Google WiFi unit's LAN Ethernet port through an unmanaged Ethernet switch (or switches). That way, they are no longer trying to send traffic over the WiFi mesh that shares the same 5GHz channel as clients are using.
I guess I'd also ask how you have your existing system placed (and whether the primary is in the center with the secondaries one or two rooms away from there in any direction where they can provide coverage to more distant clients). I'd also ask what leads you to want buy another node and where you plan to place it relative to the primary.
I hope some of this helps!
