That area starts where the lowest RPM that the engine is required to make power at.
If you are using a 5k stall (or better yet, you have a clutch) and the engine never falls below 5k, wgas what is happening at 2500???? I would give up 50 HP and even more torque at that RPM to gain it above 5k any day. If people want to think that it matters it doesn't. No one will tell the difference of 50 hp BELOW functional RPM but you'll see it in the times slips if you get that same 50 above functional RPM.