Good experiences with many FileMaker Pro hosting providers. Among others, WorldCloud and FoxTail Tech offer Citrix XenApp.
I was impressed at DevCon '11 with NimbusHosting.net (sister company of Facility Wizards). They were super knowledgeable about and focused specifically on Citrix hosting for FileMaker Pro.
We host our own files on a Xen server. Most of our users are remote. The app works very well as long as the internet line is open. Loosing connection is not a problem because the session remains active on the server. Access can be made from pc,mac, ipad, iphone.
Are you using XenApp? Who is your application hosting provider? (I'm loooking)
I have recently set up hosting with HostfileMaker.com and I am finding the service very good. You can choose the package most suited to your needs and the price is reasonable from £19 per month and an initial set up fee of £50.
I assume the service is a global one but you may need to check into it. You can test he speed of the service for your area to check the speed of the network in your area.
I hope this is of help, otherwise good luck