A slotless cache card fits in between the 68040 socket and the 68040 Chip. You'll end up with a processor that looks a like the G3 with some chips sticking out the side of the heatsink. Adds 128K of 68040 L2 cache (an 040 feature that is officially unimplemented. Thanks, Apple.). It's very rare. I think Sonnet, MicroMac, or Daystar made them once.
For a 25Mhz Quadra, a QuadDoubler (which replaces the 68040 CPU is better) since it replaces the CPU with a 50Mhz 040 and adds a 128K cache. They don't work in 33Mhz Quadras (maybe someone was able to hack them to work?)
Another intriguing slotless device was the Allegro 030 which replaces a socketed 16Mhz CPU with a 33Mhz 030. It was sold for use with IIx, IIcx, and SE/30. It won't work in an LCII, or LCIII, because these use a different form factor 030. Also very rare.
About the fast hard drive item: Has anyone tried running a 68K Mac off a flash drive? There were SCSI Memory Card readers, and an internal one here, with the low prices of flash drives (2GB CF at $20) these look like viable, fast alternatives to SCSI hard drives. Here's another manufacturer of solid state drives and CF adaptors.