Now, I'm not F3 qualified, but I can get access to several F3 books at work and if there is anything definitive which isn't remotely classified I'll clear it up later.
But I'm pretty certain that the tailerons would be unlikely to hit the tanks unless you were in Mech Mode - as I understand it the SPILS and the CSAS limit the movement of the tail surfaces under normal operation and in any case, the control surface deflection you'd need to hit the tanks at the kind of speeds when you'd be in 'batwing' would generate mahoosive G!
I was tempted to say that it's a CoG issue too, but that may not be correct, as the F3 is long enough to taxi in 67 wing (and normally does) with the big jugs on, but if you do that in a GR you might well end up doing a wheely! However, it may be also due to the relative positions of CoG and CoP that you limit the sweep. If you get one behind the other it becomes negatively stable, and results in a spontaneous reverse-assembly!
Thom