My experience with the Daystar Turbo601 in the IIci was that it was not significantly faster (if at all) than a Turbo040 upgrade in actual use. This gibes with reports that a PM6100 doesn't really give you a speed improvement over a fast Quadra. There might be some PPC optimized operations for which it is faster, but in day to day use, the early PPCs just don't gain you much over a late model 68040.
Additionally, there is a world of compatibility issues when using the PPC upgrades in 680x0 machines. Maybe not in a simple stock configuration, but start adding a JackHammer (if you want performance, you want a JackHammer) and a fast video card, and you have compatbility issues to wrestle with. It's just easier to stay 680x0 in 68K machines.
Ultimately, I sold my Turbo601/66, which I had modified to 96MHz and just ran the IIci stock or with a Turbo040. If I want 601/66 speed, I'll get PMac machine.