LO, now that is different. I can't speak for the P3/8 but the Nimrod would remain high as long as it could but once it descended it would generally remain low trading climb fuel against higher fuel burn. I don't recall being low as having a particularly high fuel penalty.
I recall a particular Cat check when I was presented with the Met charts and asked what level I would use on RTB. The standard reply would be 370.
Being given the Met charts I suspected the better answer was different. I eyeballs the charts and selected 180 - low burn to cruise, higher cruise burn, lower head wind component. My checker was surprised but accepted my logic. Later he worked it out accurately and confirmed 180 was the best height.
In short, there is rarely a one size suits all but quality every time. If the P3 was so much better than the P8, why not upgrade the P3?