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Hi Patrick,
For the optimum intercepts for an active whipamp, one should optimize it for the actual whip length that will be used. But for a circuit such as mine that has such high intercepts to begin with, losing 10-15 dB is probably not going to make a lot of difference in actual usage of the antenna. I've observed in the 10-15 dB difference for one of my circuits before it's optimized for best 2IMD performance. I have little experience for other whipamp circuits. (3IMD performance is primarily a function of the output stage current; more current gives better 3IMD performance, as a general rule.) Most of the antennas I've sold have been to LF DXers, and if they experience IMD due to strong nearby BCB stations, they simply reduce the whip length until the IMD goes away. The most extreme reduction that I'm aware of for one of my own antennas is a reduction to a 22" whip. I recommend a basic 36-39" (approximate one meter) whip to start with. My whipamp has -2 to -3 dB overall voltage gain (monopole version), and is more than sensitive enough. I live in a rural area with no strong stations anywhere around; I've used as much as a 56" whip, but could tell only a very slight improvement in sensitivity over a 36" whip. I know there are commercial whipamp circuits, such as the one by DX Engineering, that recommend use of a 102" whip, but that circuit also has quite a bit less voltage gain, around -12 dB, I believe. Perhaps with such a circuit a much longer whip is worthwhile, but not with my own circuit, in all the tests I've done.
I lived in the very-urban San Francisco Bay area for a number of years (Palo Alto), with a high level of RF from numerous 50 kW BCB stations, and used my own whipamp with no problems from BCB IMD, with a 36" whip, mounted about 10' off the ground.

So, given a choice, I recommend to try to optimize the whipamp bias for the actual whip length to be used.

73,
Steve