SP2 is surely not slower than SP1.
And it's certainly faster than SP1 with OUR stuff, because we heavily use Simconnect, and this was an area of major improvement, because up to SP1, Simconnect used to work creating an internal TCP network connection between applications, and this was both slower and less reliable, because it had issue with firewalls that, in an excess of security paranoia, like to block even connections between your own machine.
SP2 moved to a different system by default (it still can use TCP, but it's not default anymore), called "Named Pipes", which doesn't involve any network/tcp, so it's both faster, more reliable, and doesn't have issues with firewalls.
Since we use Simconnect a *lot* in our sceneries, the improved Simconnect that comes with SP2 is granting us some performance boost. AND, since we don't use BGL to store 3d objects but we draw them in C++, we are not hitten by the loss of LODs in BGL that happens in SP2, that several developers complained about. So yes, for our products, using SP2 will give better results.
On top of that, the install limitation that checks for SP2 before going further, is not an artificial one imposed by the installer, but it's a real one. Since we make *specific* SP2-only calls to Simconnect, the scenery will not work properly with SP1.
So no, there's no workaround.
The fact you are using XP doesn't have anything to do with SP1 or SP2. SP2 works perfectly in XP, it's just that you wouldn't be able to use the DirectX10 mode, which is not much of a loss, IMHO. But all the rest of SP2 features will be available. There's no need for Vista to use SP2.