My understanding is that on the Mac OS, Apple never took advantage of the IOPs in the IIfx, and just put them in pass-through mode so that operations to the 53C80 and 85C30 were just like on any other macintosh, except that the signals happened to pass through those microcontrollers. Reportedly, A/UX did make use of the IOPs.
Does the Mac OS make use of the IOPs on the Q900/Q950?
Were they some kind of 7805 variant?
Newer Xilinx FPGAs typically have dedicated DDR2 controller cells built in. For example, the Xilinx chips in the Pano Logic systems have four DDR2 controllers on board, although only two of them may be pinned out to the package.
The DDR2 buses on those chips are 16 bits wide. DDR2 chips are available in capacities up to 2Gb (256MB). A single 1Gb DDR2 chip would give 128MB of memory, albeit, only 16 bits wide, but at DDR2 speeds, one could handily string two operations together for every 32 bit access, although that would add a bit of logic.
Alternatively, two DDR2 controllers could be used, each 16bits wide to yield a 32 bit wide bus, but the controllers are physically separated on the FPGA, so splitting the 32 bit operations, to essentially stripe them across the two DDRs might be a bit tricky and/or create performance/timing issues.
Xilinx has/had a memory configuration tool that could create a DDR2 controller out of generic logic. That tool was capable of creating (IIRC) arbitrary bit width controllers, but certainly, up to 64 bits wide to access a standard DIMM. That might be a better option, but using the dedicated hardware, if present is more economical of logic.
The LC uses the edge connector for fan and/or speaker, IIRC. The LCII moved to pin headers, so you'd need to swap other LC II components to get everything connected (i.e. you can't use LC speaker and fan with an LCII board, or maybe you can since the edge connector is still there???) Other than that, the board dimensions are as near as makes no difference close enough, so the case swap should be no-problem doable.
The LCIII board moved components around, making the dimensions slightly different, so the fan placement might be different… Not sure on that one. Moving an LCIII board into an older case might prove problematic, based on photos of the boards.
Maybe relying on Cunninham's law might draw in a complete/correct answer here