As an Esbit user myself, here are my thoughts:
1) You're not going to see any more heat production out of alcohol than you will with Esbit, so that isn't the answer to cooking for more people/heating more food. If you really need to pump out lots of BTUs, petroleum based fuels, or wood burning might be the way to go.
2) At higher altitudes, I actually get quicker boil times and use less Esbit. This is primarilly because the boiling point of water is lower and it doesn't take as much heat to bring my water to a boil for the dehydrated stuff I eat.
3) As to making Esbit more efficient, what you have to do is decrease the amount of surface area of the tablet that is burning....or that is exposed to oxygen. It will then burn slower, and more efficiently. This is what we do with the Gram Cracker.
4) Since you and a few folks brought up the Gram Cracker and Caldera systems.....let me provide that context. The Gram Cracker is a 3 gram titanium Esbit stand with adjustable titanium "wings" to throttle the air flow to the tablet. The wings also will rotate vertically to facilitate burning 2 tablets at once which also might help your 2-3 person cooking issues. The Gram Cracker is manufactured by Trail Designs specifically for use in our Caldera Cone/Ti-Tri systems. These cone systems also help efficiency by protecting the rather "lazy" flame coming off the Esbit, capturing the heat and funneling it to all sides of the pot.
Hope that helps!