We own an Arbortech 160 and it is excellent for cutting everything except brick. It will cut bricks, but it becomes too slow and tooth wear will be excessive.
What we do is use a 350mm (14") diamond bladed angle grinder for the brickwork and the Arbortech on the blockwork.
btw, there is no such thing as a 10" angle grinder, they come in the following sizes: 16", 14", 12", 9", 7", 5", 4.5" and 4"
A 12" angle grinder will just cut through the blockwork, depending how thick the old plaster is. That's why we bought a 14", it cuts nearly 5".
Is this opening to be cut on a outside wall or is it a knock-though into an extension?