Emu XL-7 & MP-7 User's Group

I was planning on waiting quite a while before expanding my XL-7, but I caught a good deal on an Orbit 3 and a Planet Earth module.  What are the advantages and disadvantages of harvesting the Roms?  The ones obvious to me are save space, more gear to lug, more outs, double the polyphony, seperate processor, midi lag(is there any?).  

If all the ROMs are installed in the command station can a user preset include instrument layers from different ROMs?

Thanks