Hey Shane,
Sounds like the plumber is talking about an expansion tank. Which
doesn't make sense to me with your issue. Expansion tanks are used for
heated water since hot water expands and in a closed system water
expansion could get to a dangerous level and cause some aspect of the
water lines to rupture. An expansion tank is flexible and can expand
and contract with water pressure dilations. This however will not to
my knowledge solve your COLD water pressure issues since an expansion
tank (again if we are talking about the same thing) hangs off the hot
water lines after it is already in your house and through the water
heater.

145 psi sounds.....incredibly high. When I test copper fittings I
usually compress the lines to about 80psi and that's high enough.
I don't know about your unique situation but what I can think is
possibly someone never reduced the 3/4in copper line to your facet so
you are getting a much larger load of water at the same psi as the
other facets....just a thought.

I would get another plumber to verify the psi since the whole expansion
tank sounds bogus to me.