Shortening the upper link wont alter the static Roll Centre location, it will however limit the amount of horizontal roll center movement.
Most team drivers have found raising the rear arm mounts 1 - 1.5mm to work well in freeing the bite of the rear end.
Raising the mounting height of the arms (hingepins) has a far greater effect on roll centre Height, than changing the upper link angle (towers and hubs).
Tune R/C height through raising / lowering of the alloy Hingepin mounts,
usually keeping within a couple mm difference in height...from front arm to rear.
Use the towers for changing the Dynamic location of the Roll Centre, by lengthening or shortening the link to effect how much the Roll Centre Deflects sideways during Roll, Also for fine tuning camber gain (during suspension compression ) for optimal contact patch of the tires through the turn.
The new alloy rear hubs are nice for finetuning Rear R/C, and Camber Gain
The upper middle R/C location on the towers is optimised for 95% of conditions, changing one end from the other too dramatically will have unstabilisisng effects...always keep in check the Roll stiffness balance from front to rear...(including R/C heights)..
I find too greater difference in roll centre heights, from front to rear leads to lesser potential cornering power overall...as they fight against each other.
Tell us your thoughts, what have you tried, under what conditions and how did you feel it effected the cars handling?
sorry Im bored at work..monday Itis