The modification is using input and outputs on the CPU that are normally used for reading the buttons. In other words, using this exact modification, the SD card must be connected inside the Gameboy.

The modification is using an MBC1 chip. MBC1 can normally support 32 kB of SRAM. However, the modification is using SRAM address lines from the MBC1 chip as control lines for the SD card. This limits the available SRAM size to 8 kB. Which means no LSDj for that reason.

It's using a RAM chip as a replacement for the ROM chip, which means that the program/game needs to be loaded into RAM. This would take some time, and according to that guy, it's slow for big ROMs. (And LSDj is twice the memory size he's using.) So, it would be slow, and need to be done on every startup.

This mod also requires you to explicitly save data, so without adding a battery, data would be lost unless explicitly saved.

This mod is interesting and impressive on its own merits, but changes nothing regarding the flash cart business. Wait for derpcart instead.

Would this work with LSDJ? I don't know if the mapper is supported.

No, he mentioned it only supports MBC1. So a lot of games won't work. Also only supports 8kb ram and LSDj needs 128kb.

Still is a totally awesome mod.

that is fucking amazing.  Would the only issue be read speed?  Or would that be negligible?

It says it's quite slow, but the feature of having as much sav's as you want is really great