Thanks Yaw and Gareth for helpful discussions,
So the real world scenario we are facing now is that data are being sent via satellite phone from places where there is effectively zero terrestrial coverage of either phone lines or GPRS/1-4G.
Depending on the model, data plan and level of interference to the signal, the speed of satellite data is between say 12 kb/min and 800 kb/min (more often than not at the lower end). Cost for this is about $1-$5 a minute regardless of the amount of data actually sent.
Total amount of data across project lifespan is upwards of 100MB/month, but this is being sent on a sat-phone that is shared by many users and so time on the modem is at a premium in many senses.
I admit that I didn't know about gzip on tomcat server (and I don't know how I would implement this) but I assume that you wouldn't get a massive level of compression on these text files and you'd still be looking at the 100s rather than 10s of bytes for this data. The JSON format seems sensible though as a non-coder I have no idea how feasible any of this really is.
The ideal situation in my mind would be that (a) data sent to server is created in a more compact format such as the SMS or JSON formats AND (b) it is also compressed for sending to server.
As examples go, I freely admit that using the context of a health emergency in the most remote parts of the world is the extreme, but it is cases like this that highlight the need the most. Outside of emergency response work, our experience is that many of the projects we have facilitated for 'normal' research projects have had the same issues. The number one issue we have is data sitting on devices when it should be on servers. Most data from low and middle income country settings is being sent over prepay sim cards rather than wifi broadband, so imho every byte costs time and money.