Please clarify some of the configuration. How many users at each site are contending for their respective 64Kbps access link? Are there any other services across the link such as Internet, file servers, etc.? What is the bandwidth from the HQ site to the Internet?
Are all the backend services (Exchange and Navision) located at the HQ site and is HQ also the Citrix site? Are your end users accessing those backend services via Citrix sessions? Are the users using Citrix for Internet access?
It would seem that your end users, through Citrix sessions local to those services, should experience fast access to the backend systems hosting those services.
If they are accessing the Exchange Server or Navision Server from a local client (on their desktop on the far end of their 64Kbps link) then I could easily see how response would be slow.
To place bridgehead servers at each remote site for the sake of faster local response time would be better assessed if we knew how many users are at each site. Administrative overhead would certainly increase as would material costs and I would have to imagine that the 64Kbps link would incurr significant saturation with synchornizations and that could squelch other necessary traffic.
In general though, I would have to say that all access should be faster if done exclusively through the Citrix server as long as the Citrix server is local to the backend servers.