I started looking at the existing systems available as for today. I really like my Turnigy 9x and have modified it already to match my needs. So I would like to keep that. These are some options available as is.
Frsky Telemetry with display
The Frsky DHT-U telemetry system looks very promising. It has many sensors available already and is in fact a complete 2 way system. So you could just swap your current tx/rx system for this one. If you want speed and altitude just like me you'll need at least this set up:
* If you already have a Frsky DJT 2 way module (or DF module for futaba) you could always get the FLD-02 display instead.
** GPS isn't the cheapest sensor anyway. Many other sensors are available for less, think of barometer (altitude) for 10 USD, temp for 4 USD, etc. For some reason the gps didn't provide altitude information!! Well after checking the official frsky protocol information it seems to be sending this information back to transmitter but it doesn't show on the display. So for altitude you'll need the barometer anyway or you'll have to wait for a firmware fix.
For those who only want the voltage information with optional temp and amp draw there is a cheaper telemetry system available from quanum (hobbyking). It doesn't include a 2 way transmission so it only works as a surplus on your current transmitter system. If you look around for a buddy code you can get this one for around 45 USD. Please note that by default this system only provides voltage display. To get temp and amp draw you need to extra module. The V2 receiver is ready for the 2 extra signals (the kit just doesn't come with the sensors).
Its limited to these 3 readings (voltage/amp/temp) but if you only buy the rx/tx kit you still have 2 spare channels. With some knowledge of electronics you might be able to update this system by using your own sensors on these spare channels.
Custom Frsky Telemetry Project
If you do intend to have your own sensor data transmitted or you already have another method in mind to display the transmitted data and you're not afraid of some work the Frsky system has many more valid options.
You could always buy the original DJT module (for JR modules like the Turnigy 9x, for futaba choose the DF module instead) and a 2 way receiver or get the combo pack right away. You're then free to transmit any data you want and use any display available.
I personally would prefer a DJT module (backordered one already) and a gps via sensor hub in the plane. Than I'll add a proper bluetooth connection to my receiver to display everything on my android. This way I can display the data however I prefer. Once I received my DJT module and get something working I'll prepare some information on this blog.
For those wondering how to replace the existing Turnigy module with a valid JR module.
Some (French, translate if needed with google) information on the Quanum system.
Great (Dutch) project info for custom sensors on the quanum system.
Frsky telemetry hub & sensors topic on rcgroups.
BMP085 barometer sensor information