RR.. I have the F1's on my front and the Sport maxx on the rear but havent experienced anything of the kind. Took about 100miles to bed in and I havent looked back since... they would piddle all over the previous sport maxx I had on the front. Understeer??? what understeer??
Yebut, the SportMaxx are shyte, and if can not really determine any difference in grip, particularly in hard cornering, between front and rear axles, the F1s arn't really an improvement over the SportMaxx!

I'm in a similar situation to you, having directional SportMaxx on one axle and the asymmetric Mich PS2s on the other - and I can state there is a considerable difference in cornering grip, traction, braking, and wet weather performance between axles. Put the PS2s on the front and the ShyteMaxx on the rear, and I lose the back end under cornering (particularly if there is a bump mid corner), and the ABS kicks in on the rears. Swap em round with the Craplops on the front and the PS2s on the rear, and the rear end sticks to the road like sh!t to a blanket during corners, when the fronts will push wide, and literally jump wide by about ½ a metre if I hit the same bump. The Craplops don't have particularly good traction in the dry (they may be just about OK, but compared to the supreme traction of the PS2, there is no comparison). The ABS kicks in on the front end. And in the wet, the SportMaxx, just like all directional tyres suffer with scary aquaplaining after just 2½mm of wear reduction (that means 5mm remaining).
And just to be clear, when I carried out these comparisons, the PS2s were brand new with 7½mm of tread, and the SportMaxx (being the original rears) still had 7mm tread - so the comparison between them is about as fair as you can get.
I tried similar experiments on a previous front wheel drive car too, using the 1st generation directional Mich Pilot Sport, and (when the orginal directionals became discontinued, before the PS2s became available in 17" sizes), I got an axle pair of the latest asymmetrical Mich Pilot Exalto PE2s. Again 7mm left on the PS directionals, and 7½mm on the new PE2s, and never experienced any such handling concerns (apart from aquaplaning on the directionals when they had worn down to 2-2½mm, which, as I've said is a common problem with directionals, and is just one reason why manufacturers are scrapping directional tread patterns) - irrespective of weather the directionals were on the front or rear.