Chris, I have found the Spitfire flies wonderfully at low altitude- I just make sure that my boost is around 0 if I'm doing a low altitude flight. Sure you'll be under 200mph, but the engine should stay cool enough, and the radiator flaps should help if you need them.
Have you had a chance to fly with all three different props yet? Each one makes the Spitfire like a totally different plane.
You will LOVE the B-17, but don't overlook the P-40, it may just be the best one to date- more challenging than the Spitfire (to me), and flies totally different too.
And of course there's a brand new P-51 flying low straight towards us, hopefully to be released very soon. My expectation is that the P-51 will be the single most authentic warbird ever created for a flight simulator...
You discovered A2A at a very exciting time!