you gain aprox 2.8 degrees for every pound of pressure, so a 15-16 pound cap
will raise the boiling point of the coolant 45 degrees  .  The computer
system expects to see a certain operating temperature.  If the vehicle does
not reach a high enough temp the computer assumes the engine is cold, and
the fuel management system runs on a preset set of parameters called open
loop, i.e. it doesn't read from knock sensor, it retards the timing, it
ignores other sensors.  Its basically like driving with the choke on on a
carb.  In the early days of replaceable prom chips, there were chips
designed to work at 180-190 degrees instead of 212.   Fuel mileage still
suffered.  The whole idea of running at higher temps is to get more
efficient burn of fuel.  The engineers would love to see 250, but pistons,
heads, and gaskets aren't there yet.
As stated in another post, there are surfaces internal to the engine that
exceed the temp you see on the gauge.  The boiling point for the coolant
must exceed these temps, or we have problems, remember early 400 small
blocks before they added steam ports to cylinder block decks and the heads?
(400 blocks the cylinders were siamesed, ie there was no clearance between
the cylinders in the cooling system)

Whitelightning