Of course it depends on the track
on a reasonably bumpy track with a few jumps i have been running 1.5 straight pistons in the front with 40wt oil and 1.5 conical pistons (with the larger hole at the top) in the rear with 40wt oil
On a reasonably flat track i have been running 1.5 straight pistons in front and rear with 50wt oil in the rear and 60wt oil in the front
I would have to say that on a bumpy track with or without jumps the conical pistons in the rear definately improved the handling,as the upstroke is quicker but has more pack which stops it from bottoming out and the downstroke is slower to stop the rear from lifting which seems to settle the car quicker over the bumps and ruts (on the bench it feels way to soft but on the track its awesome
)
I always tend to run the front stiffer than the rear by using stiffer springs or using the straight pistons,this seems to take a bit of the steering snap away and also seems to be more precise into a corner and stops the front from squating in the corner
The answer to the hole diameters is that the larger the hole the quicker the oil passes through the holes which i feel makes the car more consistent,especially over the bumps and ruts
On the bench the 1.3 with 35wt wont feel much different to 1.5 with 60wt,its when on the track you feel the difference
(as explained above)
Hope this helps