600cc would be a total disaster, and we'd be left with one manufacturer - Honda. Smaller engines would mean a race for horsepower, which would mean a loss of torque, which would mean a race for electronics to control the engines. It would be exponentially more expensive than the 800s. Smaller engines place a premium on horsepower - the difference between 150 and 160 hp is a hell of a sight more than the difference between 250 and 260 hp. The factories would build highly-strung, peaky bikes that needed a shitload of electronics to control. With no torque, it will be all about corner speed again, and there will be more crashes, more highsides, and more injuries. The racing would be even more processional than the 800s, as mistakes would be mercilessly punished with no chance to compensate for them on the way out. You would be able to predict the races after qualifying, adding a random correction for starts.
From an engineering point of view, it would be fascinating. From a racing point of view, it would sound the death knell of MotoGP. When I think of 600cc four-stroke MotoGP bikes, I think of the Honda RC166, the inline 6 cylinder. One of the must stunning motorcycles ever produced, and the bike that effectively caused the cylinder limits to be introduced. An engineering miracle, and a sporting disaster.