|(L) Original pattern, (R) Longer crotch
Sorry for poor photo quality.
I had a pair of underwear cut in pink cotton jersey before the crotch length revelation. So I cut a second pair from the same knit with a longer crotch piece (~4.5cm). Then I sewed them up to compare.
Conclusion: Add 1-1.5cm to the original crotch length and they should be great! Also, I've been using two-way stretch fabrics while RTW undies use 4-way stretch. For best fit use 4-way stretch, or increase the ass of your pattern ;)
Now that I have some new undies I retired some old pair, and one or two will be cut up in the name of research. Onward and upward!