You will find softdep performance noticeable whenever you issue a large rm(1). It has other benefits as well; see the links in FAQ 14.5.
I use MFS regularly for /tmp; I have used it also for /usr/obj and /usr/xobj, but no longer do: /usr/obj has now gotten larger than the upper size limit for MFS on one of my architectures, and workarounds are not worth the effort. Plus, because I am making releases, I sometimes want to keep /usr/obj between reboots. Was the elapsed time noticeable? yes. But we're talking hundreds of thousands of file accesses during builds. Was it enough to keep MFS? No.
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