Can you judge the image quality of high-speed inkjet presses by looking at specifications?

That's difficult to do. While resolution is often described as dots-per-inch (DPI) or sometimes nozzles-per-inch (those two measures aren't the same), it is but one of many factors influencing quality. The number of drop sizes produced and the size of the smallest drop also have an influence. Other factors include the consistency of drop sizes, accuracy of drop placement, ink and primer chemistry, and RIP performance--which aren't in specifications. Comparing samples of your work is essential.