Motor KV in MBX-5?

[SilentMonster]

So far I have decided I am going 4S, MM. I'm pretty sure I am going with the Lehner line due to the Neu/MM cogging issue. I am a little confused(read a lot confused) with what KV motor I need to run. I am using the motor calculator that BrianG has made. It tells me to run a 1940/8 with a 15/46 gearing. Top speed is at 40MPH. That motor will spin 30543 rpm's at 3.7 volts per cell.
That all makes perfect sense to me except I have read several times I need to shoot for close to 40,000 rpm. If I change the pinion to 12 that puts it right about there but then recommends a 1940/6 which from what I gather is a little to hot of a motor to be running and isn't what I see on other buggies here.
I understand the lower the turn the more efficient it will be but isn't there a point that gearing a lower turn motor will offset the efficiency due to over gearing?
I'm setting this up to race so there is no concern for speed runs. What am I missing? Also would I gain anything besides weight if I went with the 1950 series or is that just over kill for a buggy? Stephen.