The FC is a fine platform for racing. I have one myself.
Having said that, I must point out that the FC has some non-trivial performance handicaps. The most significant of which is the all important suspension. McPherson fronts and trailing arm rears are not quite the state of the art. They can be made to handle well with the proper setup, but will never quite be as good as a true double A-arm setup (ie. the FD).
The reason that a lot of us run FC's on the track is that it will be a very good and competitive car in its class
. Or else we just like the FC as a platform. Performance parts are reasonably priced (thank you Mazda Motorsports) and for cheap(er) fun racing they are hard to beat.
Bottom line - what are your class restrictions? Are you looking at a serious commitment to winning a class series trophy, or are you looking at just getting your ya-ya's out driving fast in a circle on weekends? (there is no shame in the latter). You can build an FC into a truly great track machine. You can get the car running fairly cheaply and constantly improve the vehicle as your driving skills improve. Shells are cheap in case you do end up balling one up.
But if you want something that's gonna go out there and eat up Porsche's and M3's for lunch, it may not be too much more expensive in the long run
to start with an FD shell and build from that. Initial cost of getting a running FC track car may be less. If you got the coin up front and aren't limited by class restrictions, the FD may well be a better starting point.
Do a search back through this forum. There was a great thread on building up an FD track monster, as well as great advice on getting the most out of an FC on a budget.
Good luck and keep us appraised.