I've been playing with Tyre in the last couple of days too. I used it to convert TT itinerary files into .gpx format.
When the gpx file is opened in Mapsource you get a track log of the route, and a 'route' with all the waypoints but the route itself needs to be 'recalculated' to lock onto the roads. I had to do a bit of tweaking to get the calculated route to follow the track log.
I copied the final gpx file onto the SD card of my 550, imported the routes and all looks fine in the preview screen. The 550 did a recalc as it imported. The only PITA is that the route preview screen is plastered with brown flags showing all the waypoints.
I also imported the track logs created by Tyre in the gpx file, and interestingly these appear in the list of routes on the 550. I haven't explored yet whether they are truly routable.