This sounds alot like what I've already created in the past which LS, SF & LV were at war with each other. It was a successful hit in the past that ran my server from 0 players to about 35+ or so within just a short month, but it was way back before RP Servers became so popular and before the GF GM was even leaked. The reason I decided to shut it all down was because of switching over to normal RP because my old server wasn't RP at all...
However, only recently I've decided to start it back up which RP will play a much larger role in my past history with the server. The combined TDM/RP will certainly make things interesting because after a while, normal RP just isn't as appealing to me as it once was (Get bored with it), so blasting away another team (More physical action) while still in bounds with some RP seems to be a great solution
Drawing the line where your TDM meets your RP can be tricky though, you're going to have alot of players that are likely to treat it as a DM killing even thier own team mates and whatnot. But putting up some RP Quiz or enforcing a proper First_Last name format is likely going to keep everyone out, even your RPers because of the confusion. I for example, wouldn't go too deep into the RP side, but just enough to keep it all tolerated even by non-rpers, they will then at least be given the chance to slightly learn a bit of RP :P
If you do this correctly though, I believe you can still pull off a nice player count though. Mix in a few RP Elements such as civilians, criminals, cops, medics, etc. who may choose to stay out of the war or jump straight into action if they chose to do so (Medics healing wounded soliders for example). Probably ideal to throw in some turf fighting as well, gives them purpose as well as something to actually fight over.
I'd team up with ya developing such a server, but I'm currently done with SA-MP
(GTA Related, I'm probably going to focus on a GTA V MP when/if it becomes availible etc.)