The advantage of putting a rough poly in the cross, such as ALU Power rough or Solinco tour Bite diamond rough, is that you also get additional "string to ball" friction. Where a smooth poly will allow great main string movement, those smooth crosses slide laterally over the ball without gripping it at all.
but then again, it all depends on how you generate your spin. If you swing slowly and rely on string to ball friction, then using the roughest strings possible will help most. If you swing very fast and rely on string elasticity/snap back motion to apply spin to the ball, then you want your mains moving freely as much as possible.
As to tension differences, I don't know what you mean by "exc.". my rule of thumb is: homogeneous string bed: string the same tension in mains and crosses. gut/poly hybrid in standard 16x19 or 18x20 frames, string crosses 4 lbs looser. spin effect frames, i.e. reduced cross string frames, always string same tension mains and crosses regardless of the strings.
For ALU power mains plus x-1 biphase on crosses, what you're going to get is a slightly more powerful string bed than a full ALU power bed. However, you're going to likely snap your x-1 biphase crosses before the poly strings die. lastly, you're not going to get much if any mains string movement: so for topspin you're relying on string to ball friction only.