I am looking at developing a timing board here in New Zealand, but it will be used in the US for an event (And other countries hopefully). I am considering using Flutter to link multiple timing boards - I need 500m range from the start/finish line to spectator area.
I noted in your update you don't need a hardware filter anymore, so will Flutter be switchable for different locations?
I am then also concerned about shipping the finished timing boards to the US.
I shipped a PC to US for work with a WiFi antenna card and US customs asked for a FCC 740 Form (pdf) Statement regarding the importation of RF devices capable of causing harmful interference to be filled in. I had to go the WiFi manufacturer's website and include their FCC attestation.
I assume you will get one of these from your testing that you have just completed.
Fortunately I can do a lot of the software and hardware development on standard Arduinos so I can afford the time to wait for Flutter to be publicly available.