Frankly I am surprised you can have that many users work from such a distance over 20 Mbps. That is actually really impressive to me that it is usable. I would suggest turning off multi-monitor support. I have my PCoIP session bandwidth floor set to 1024 in GPO which has helped with frame loss but in your limited bandwidth situation that may do more harm than good.

I also recommend Blast if it works for you (doesn't always play nice with GRID cards though so I don't use it when those are involved). I use View 7.5 and don't need HTML on for Blast to work, you can't turn it on in previously made desktop pools so you will have create a new pool.

I don't know how much web browsing is going on but you may want to suggest it all happens though the VDI since then that is just using the bandwidth at your main office to go on the web instead of browsing on the local machine (if that is what they are doing) in addition to using Horizon View at the same time. Then it wouldn't matter how much the Internet is being used as long as your main office has bandwidth. I am having to make some assumptions but it seems like that would help.