Applied Engineering QuickSilver
The Applied Engineering QuickSilver card is a cache/FPU expansion card for the Macintosh IIsi. A system control panel called "AE Cache" is required to enable the cache features of this card, but the MC68882 math co-processor should just work automatically. The card features 32K 25ns static RAM cache.
While other FPU cards for the IIsi offer a Nubus slot, this card appears to feature another PDS port. This should let you use SE/30 PDS cards though you can expect some compatibility issues due to the IIsi's 20MHz speed.