Far as I know the Orbit was kind of an "LE" version of the Extreme Lead and is not compatible with the P2K/custom ROM's; Orbit doesn't have an expandable or interchangeable ROM.
On the other hand, you can put, I think, either 2 or 3 ROM chips in the P2K-series platforms, so your P2K could have the stock Composer chip (basically GM level variety), and also at least one other -- Xtreme Lead, Mo' Phatt, etc., depending on your needs and the slant of your musical tastes.
The modules are a terrific bargain if you're just looking to expand your sound palate. Lately, you can find the final issue of e-Mu rack gear, ROM's included, for what you'd pay for the ROM alone -- $200 or less.
For just ROM's try EBAY - Search "Emu ROM". For used racks try Roguemusic.com, or Guitar Center's used instrument section.
Also
http://www.theavenuesinstruments.com/emuroms.html has pretty much all the ROMS (slightly pricier than E-bay), and is from my experience very reliable.
BB