you probably wouldnt notice any difference at all and for the money you would be spending it would be better to just buy a 4mm longrod crank 115 rods and getting domes cut for it but if u do that you will also want to have your cylinders ported to match up your port timings also u will need to run blaster pistons which start at 66mm
OR............
you could do a spacer plate found here:http://www.ebay.com/itm/Boss-ATV-Racing-BANSHEE-YFZ-350-STROKER-SPACER-PLATE-ALL-YEARS-4mm-race-engine-/181240219798?pt=Motors_ATV_Parts_Accessories&hash=item2a32c24896&vxp=mtr
to raise the cylinders up then there would be no porting required but you would still have to run the blaster style piston which is a 795 series wiseco piston kit found here:http://www.ebay.com/itm/Banshee-66mm-080-4mm-4-Mil-Long-Rod-795-Wiseco-Pro-Lite-Pistons-Piston-Pair-Set-/251358138820?pt=Motors_ATV_Parts_Accessories&hash=item3a861cd1c4&vxp=mtr
most of the guys on here will tell you u should not run the spacer plate but it is an option
my opinion is also not to run the spacer plate get a coolhead and have a builder cut you some domes you will like it much better being able to change your domes out to play with your compression
hope this helps
wes