From my experience there isn't much performance advantage by running an SRx/DRx over an SLC/DLC. They should basically be identical except for cache coherency circuitry which makes them compatible with old 386 motherboards. They also have some kind of "pipelining" option that can be enabled, but I'm not sure how reliably it works or if it makes that big of a difference. The clock doubling doesn't really seem to give much of a speed boost on these chips either, because it's speculated that you need a larger L1 cache to take advantage of it. I found my DRx2 33/66 to be marginally betterthan my DLC-40 in integer performance, but worse memory throughput. In the end they seemed more or less equal to me. Unfortunately I sold mine many years ago and do not have benchmark results to share.

Now a TI486SXL with clock doubling may be another story.

Fasmath CPUs should work fine with DRx2, unless yours is a very very old model. The speed rating on the FPU should be greater than or equal to that of the CPU, but sometimes you can get away with overclocking a little.