Kevin,

 

The IO-550 has a fairly low fuel flow above 10K, compared to a TSIO still pulling high power.  The original ES and IV had 3/8" fuel lines and several 90 degree fittings that were quite restrictive.  The newer aircraft have 1/2" lines and many builders have modified the fuel system to reduce restrictions.  I flew my ES for the first two years without turning on the boost pump in flight, never.  I live in CO with cooler temperatures, but the SO CAL guys have had a few vapor lock incidents due to high fuel and ambient temperatures.  I would bet that most of the ESs could fly all day above 10K without the boost pump.  I think it's in the procedure as a precaution, not as a necessity.  Like I said, over 100 hours without ever flipping on the low boost in my ES.

 

Mike Easley

Colorado Springs