The way that he speed increases work at the moment are logarithm rather than linear; this is something we will change with he upcoming 4.0 version. So, to answer the direct question, you'd need a mainframe to tell the difference between +9 and +10, and even that might not work. The difference with just +1 is a 100% increase in speed, +2 is even more, etc. The speed increases will be made a lot more subtle and helpful when we switch to unity, I really don't like how it is now.
The game speed is something that varies based on the scenario, mainly how many ships in all there are and what they are doing. We have had folks play with -- no joke -- 300,000 units on a quad core overclocked to 3.5ghz. Most smaller maps are just fine playable on the minimum system requirements; and most larger maps are fine on the recommended requirements. The game now has some stuff in there that limits it to about 175,000 units, which keeps the upper bound from getting to crazy, too. Most players that were hitting those sort of numbers had 30+ hour games or even twice that.