Yes efficiency drops. But don't let this confuse you. It's just saying the pump isn't running at it's optimum efficiency (gallons per amp). It will still use less power but a smaller pump could do the job with even less. This why a lot of research should be done when buying pumps to insure you get the "right" size for your application.
I wouldn't worry about heat with the PSI range we use in this hobby. Your not going to notice a heat build up until you get up in the hundreds or thousands of psi ranges.