A quick B-5 Leslie vs IK Leslie test.
I used a stock setting in B-5, no adjustment to Leslie. Then I ran the same organ DI out to IK Leslie. Only adjusted Leslie speed to compare to B-5 Leslie, nothing else was changed. There is virtually no difference. I have enough controls on both to adjust to each other, so maybe a little eq, and some attention to deceleration speed would make them damn near equal. Again, this is just a quick, out-of-the-box test. Click the play button and both will eventually play.