In the past uelastix was running really smooty in a Raspberry. I still have and operate uelastix in a RPI B with 512MB RAM! To support that thought in a RPI 2+ i am running Centos 7 with asterisk, samba server, dlna, web server,... So it can cope quite well on that.
Uelastix runs as charm, but it is outdated. So it is a nice idea to have Issabel in RPI. I could help with testing if there is lack of hardware since i have both RPI B and RPI 2+. Probably i will have a RPI 3 next month. Let me know if there is room for development for RPI, please.
I have given a try to raspbx, but i did not like it. Uelastix in the past and Issabel now, do things a lot easier and better.