I have the SPX8 fitted in to my HB Ve8, If you run 2 normal stick packs to make 4s your ok. I do this using 2x Trakpower Dark 5000mAh 40c batteries.
There is a wire coming from the SPX8 with a switch that connects to the bridge wire between the 2 stick packs. Because you are taking a negative referance then the switch wire coming from the bridge wire is 7.4v.
When the system is connected in this way it does not unbalance either of the 2s packs (LRP designed it to work this way), if you are using a proper 4s single pack then it is not recommended that you connect the SPX8 to the balance port as it will unbalance the 4s pack, due to the reciever and all its devices draining one cell of the 4s pack unevenly.
Hope this explanation makes sence
You are right with the weight and size of this speedo, its so small it allowed me to make the HB Ve8 into a saddle pack like the caster. This turns it from a cool car to an awesome perfectly balanced car
