njee – look at the angle of the bolts – of course there is shear forces. However the bolts in both set ups do not take shear loads unless they are loose – what happens is they clamp the faces together and the friction between the faces takes the sheer load.
Contact surface area is greater on IS as there is no slot.
IS is simpler and stiffer as there is one less bolted joint ( 160 front / 140 rear excepted) and in all sizes the joint is better supported as the mount is not slotted. Similarly IS is lighter as you have two bolts not 4
IS mount is a fiddle to set up the first time then never need be done again.
Also check the post above about the hope method of centring pistons.
go an have a real good look at both