Thanks for your input guys, however I just want to note you guys are working out the speed per person. This is 10Mb/s uncapped for each user, so if the network is idle, someone will be able to use 10Mb/s.
Again, I know it isn't great but 10Mb/s up, 10Mb/s down, when all they are doing is really sending emails, viewing pages, etc is perfectly capable.
If we can resolve this .PST issue causing all the bandwidth problems, we'd have no issue what so ever. All files get synced as they get edited. A typical word document is around 30-100KB.
Thank you for pointing out that buying bandwidth isn't the solution.
We got in contact with HP, and they've since added a function to read the PST's and upload the marginally small edit. This has since solved our problems.
We sit at around ~30% Utilized network per site. A lot better than around 96%.
Thank you all who have contributed.