Yes, I always keep v-sync on. You'd theoretically be getting 48, but only like 2/3 of the screen would get it and the bottom third would be cut off on the last frame due to your screen refreshing at an in-opportune moment. v-sync simply makes sure you never get that tearing, and even though it seems to "limit" your frame-rate, really it just keeps things proper looking. But for testing purposes, you'd definitely want it off so you can see exact performances like you want to test above.