Thinking about this more and talking to a NICET IV friend about it, I actually think the IP receiver (whichever, I only have experience with the VisorAlarm) is the absolutely way to go in this case. Here's why:
1. A lot of the IP dialers have multiple inputs, which would allow you to essentially just remove the contact inputs that you're using now and land them on the dialer for fire, trouble, supervisory, etc. If that's all that's needed, it's nearly universal in what panels you can support.
2. However, if you need point ID support, you'll get multiple format support from the receiver. So for example the VisorAlarm will support Ademco, Radionics, Sur-gard, and I think more. That's going to cover a whole lot of the industry for point ID.
3. IP dialers have been in use for years now and they're fairly cheap and reliable. They're fully supervised, so if the network connection goes down, you'll know. I think the it's only something like 30 or 60 seconds before the receiver must report trouble according to code. The company I used to work for switched to mostly IP dialers in something like 2007.
4. By contrast, a proprietary fire network (Notifier Net or Gamewell's FocalPoint) has to be backed up to meet UL864, which if I remember correctly (it's been a few years) means 24 hour UPS. I ran into this in a large fire project back then which required a UL864 UPS to the tune of about $7000 unexpected dollars...
The fact that you can send communicator signal over the internet and not require the same backup is kind of bizarre, but that's how it's written.
The short version is, I just don't think any other method is going to scale very well at all on a 300+ building fire system. The receiver is going to be most flexible and likely lowest cost and meet the goal of the project.