It should work like that already.
That's only true for attack_time = release_time because auto shapes will be the same in that case, too. However with attack_time != release_time attack and release behave differently enough to settle far away from the theoretical target.
Setting shapes = 1 and peak mode = release_time/attack_time or shapes = 500 and peak mode = 1 seems about to work.
I've only tested analog RMS mode with auto shape on so far.
Here are plots for easier comparison. They show the level computed from input and output (samples vs percentage). The used signals are sine waves with amplitude 1 and frequency as stated below, so they all should've produced the same output
. The SB compressor uses threshold = -20 dB, ratio = 2, attack time = 20 ms, release time = 60 ms, knee = 0 dB, mode = RMS and auto fix shape = on. The rightmost plot is from a model compressor.