I've seen the Mustang fly, it's pretty quick, particularly so if you use a LiPo battery instead of the stock NiMh battery pack. If you've gotten really good with the Super Cub it might be worth a shot, though. What the heck? It's only money.

The only other thing that I can think of that might be a good stepping stone would be something like the Phase 3 Swoop RTF from Hobby People:

http://www.hobbypeople.net/gallery/107116.asp

Flying wings are pretty durable, and the Swoop should be a bit easier to fly than one of the warbirds while still getting you used to higher performance flight. It's a tough call though, there may not be that big of a difference between the Swoop and a Parkzone warbird. If you like the Swoop, try one before you buy a warbird, otherwise go straight to your Spitfire.

Let us know how you do with your warbird.