Im running FC24 with stock rooted EL29 ROM and Kernel and it seems to be doing well for me battery wise.
I dont think we will see a whole lot of data improvements until Network Vision is completed or at least launched. Im in Dallas, TX, one of the first launch cities and hoping data speeds on 3G will improve and stay improved once NV goes live.
I think a lot of people who flash these modems and see a placebo increase or decrease due to traffic loading on the tower around them.
I have been flashing these modems with my stock EL29 set up sonce FC07 came out. I would see sporatic improvements but also the occasional tower that was over loaded and gave me horrible data speeds. While i do think they are more energy efficient and have fixed GPS issues with older modems, i think we wont see the full capabilities of these new modems untill network vision is rolled out. Combine that with the fact that maybe there are some tweaks in the kernel for 4.0.3 that lets these modems run better then say 2.3.6.. Thats another thought also, maybe the people who are seeing a big improvement with FC24 also are on 4.0.3 or something and the kernel is helping. Just my guess though.
Im happy enough with my set up to allow me to wait for the official ICS drop before i wipe my phone and install ICS from scratch and install my apps one by one.